Guys, i want to thank you very much for your advice, it's been a pleasure to read as i am returning from Easter weekend!
Indeed i forgot to say with whom i'll be travelling: this time it will be just wifey (my favourite buddy) and myself, this is our rare occasion to travel without kids, just for diving 
So many choices, so little time! I have not been to French Polynesia, but I have been to the others you list. Here is what I can offer.
Solomons: Great diversity of sites, mostly easy conditions. If staying on land, go to either Munda or Gizo. Grand Central Station and Kennedy Island are both great shark dives, mostly for nurse, grey and bronze whalers, and reef sharks. There are other dives such as Uepi where you might see hammers. Eagle rays are common sightings, mantas less so. Fantastic reefs and walls. Also the easiest and most diverse wreck diving in the region
Fiji: Large pelagics are less reliable at most sites, except for Beqa Lagoon where they feed them. Get off the main island and go to Taveuni or Savusavu. Better to go with a liveaboard if you can, especially around Bligh Water. No easy wrecks.
Vanuatu: Diving around the main island of Efate is not great. Espirito Santo is the place to go, but the main attractions there are the wreck of the Coolidge and Million Dollar Point for wreck diving. Reefs not as lush as the others.
Tonga and Niue: Fantastic places to go for humpbacks in the season, but both are snorkel only not scuba. Diving around Niue is unique and well worth a visit, Tonga less so
Cook Islands, New Caledonia, Samoa: Not brilliant diving but lovely to visit
Kiribati: Forget it for diving. Pohnpei is not great either. Very little to no support in either.
Yap: Not on your list but should be, for mantas, the best in the region, though it is far from Sydney
PNG: Also not on your list but should be, especially for liveaboard.
You don't mention if you will travel solo or with family, that will make a big difference. If so, it's tough to beat Fiji which has a wide range of resorts and activities to keep non-divers happy
Don't overlook Australian diving either!
Many thanks troporobo
You truly have visited loads of these places and it's great to get your comparative feedbacks.
From your reviews it looks like Solomons could be a very good match. Or maybe Fidji, i still can't decide whether i'd be keen to go to a shark feeding place.
Whilst i am totally OK for dive operators arranging it and their customers enjoying it (after all, in some ways it protects the sharks) i am wondering whether i'd have that artificial aquarium feeling... or whether i might be uncomfortable to get around me lots of shark coming close just because the smelt food 
PNG i thought about it but didn't include it because i can picture myself living in South East Asia for some time in my life, and then it will be in easy reach. I am really looking to try one of those far-away destinations which would be tough to do when i am back in Europe or still far from central APAC.
Thanks for the tip on Yap, i didn't know and am penciling it (for later).
For the good places whale-wise, i am guessing my target time (Feb-March) isn't good, and i can't change it.
Australian diving: giving it some thoughts too.
For instance North Solitary islands or Lord Howe Island caught my attention, but not sure how these compare to Solomons or Tahiti for big wildlife.
I would say you should look at start at home on the Great Barrier Reef. Most liveaboards will do the Cod Hole which you can get within touching distance of large potato cods and some nurse sharks. You can see some pelagics around the pinnacles but they are rare. If you want the shark experience you have to do Osprey Reef.
I did Fiji in February, both land base and liveaboard and absolutely love it but its not the place for large pelagics, mostly macro and reefscapes. I have done Beqa lagoon and you can get close enough to touch tiger sharks, lemon, tawny nurses, and bull sharks but again it is a feeding site as mentioned above. There are other great dive sites around Beqa but no pelagics. You can see pelagics in Bligh Water but it is mostly in passing, we saw Mantas but again it was for a few seconds and they were gone and definitely not close enough for a good picture. You can get close to a potato cod and some reef shark and white tips in Gau island during your liveaboard. I absolutely loved Naia if you can afford it.
Check out my Fiji shots:
https://flic.kr/s/aHskxnfKaC
First of all, kdgonzalez you have some cracking pictures in your Fidji album mate!!!
Like i said above, i need to make up my mind on whether i am ok to dive on shark feeding. Any safety concern to have with these?
Really appreciate the comparative feedback between Bligh Water and Beqa Lagoon, i've been in such places where you "see" but can't photograph the big ones, good to know.
Unexpected advice on the Great Barrier Reef: we dove there 11 years ago over a 3 days liveaboard (pro-dive) and love it. However it was our first warm water/coral reef experience so i honestly wouldn't be capable to rate it against memories we got in the following years (komodo, sodwana bay, sipadan, egypt).
But my first question would be: what about the coral bleaching? Isn't it affecting the diving on places you've recommended? If not, can you recommend liveaboard duration & operators to get there?
Check out Beqa Lagoon resort I took my wife and 3 kids there in December. Great set up for familys kids did scuba and the diving was really good large Tiger sharks about 6 of them right on the fisheye dome port. Healthy reefs with loads of soft coral and turtles and a couple of wrecks. Also there is a house reef for macro found loads of cool stuff.
Tonga or Niue for Humpbacks French Polynesia for schooling sharks or Poor Knights Islands nz for large schools of fish and groups of very large stingrays similar water temp to Sydney. Southwest rocks for Grey Nurse sharks enjoy this part of the world loads to see
Thanks Crayfish, we did dive South West Rocks recently and LOVED the experience.
Feb-march won't be right for humpbacks i reckon, but i am penciling in Poor Knights Islands which i didn't know about.
Again, wondering about the shark fed in Beqa lagoon. Keen to know more about how the diving is organized, how long, how do the shark behave, whether you feel in danger.
Schooling sharks in french polynesia sounds interesting, getting close enough for photos?
My preference would be un-shy big animals that come reasonably close for photos without being fed. Yes i am asking a lot, but hey only 1 dive-dedicated vacation over ~2 years, gotta get it right!
I can recommend Ningaloo Reef off Exmouth, Western Australia. Lots of Martine life in abundance and diversity. Better, I thought, than the Great Barrier Reef. And there is whale shark watching too.
I liked too Moorea in French Polynesia - a few minutes hop from Tahiti. Lots of reef sharks, lots of life.
Thanks TimG!
I did snorkel with mantas 11 years ago (not yet a diver) at Coral bay and it was a magic experience. How is the diving from Exmouth?
Day trips? Liveaboards? Any good shore dives?
I remember (11 years ago...) seeing small reef sharks and turtles swimming in 1 meter deep water by the shore, at Coral bay. That's the kind of experience i'd love to get again (and now i have underwater photo equipment...).
Great question, and great region to explore...
As background, I lived in Sydney twice, once for high school (w/my parents) and then again for work (when our kids were young). We've also taken a number of trips throughout the region in recent years.
The first thing I would say is that if you don't take your family to Fiji you are going to be missing out on a great experience. Fiji is one of the most family-friendly destinations in the world. Fijians are also known as some of the friendliest people on the planet (and they love children). Almost all the dive resorts that allow children will have a nanny program where for very few $ each child will be given a nanny for the entire stay. Our first trip was when our daughter was 18 months, we went again after our son was born, and then again when they were 4 and 2, (and since they both now dive, we've been back as a family of divers). The nannies are great - and by the end of the week our daughter was playing with several of the local children in the village. A great experience... (and very safe).
Several great recommendations have been made already, so I'll limit this to a few of my favorite locations in the Pacific.
1. Solomon Islands (either Uepi Island Resort or the Bilikiki): We took our kids (when they were 12 and 10) to the Solomons - they had a great time (snorkeled with reef sharks every day off the dock). The diving in the Morovo Lagoon is some of the best diving anywhere in the world (no other divers, very healthy reefs), mantas, sharks, etc. I have also been on the Bilikiki (w/out the family) - also very good. In general, for the Solomons you are going to get the best shark action where there are fairly strong currents - the dive operators will put you in a spot to get some good shots, but you want the current to bring these animals in closer.
2. PNG (Febrina): outstanding diving. As a bit of an old-timer, I can say the number of sharks is much less than what it used to be like, but it's still very good for large pelagics. There are still sharks on many of the most famous dive sites, but just much smaller in number (and it feels like less likely to see some of the larger sharks, like tigers). Having said this, PNG has some of the most beautiful reefs anywhere in the world (similar to the Solomons, with even a touch more diversity in critters).
3. French Polynesia (Rangiroa & Fakarava): If you want sharks (and not just lots of sharks, but a chance to see some of the larger species) then this is a great location. The passes in both Rangiroa and Fakarava can have huge schools of reef sharks (esp. Fakarava), depending on the tides. In Rangiroa you can find yourself drifting through the pass with several large species of sharks (I have seen a tiger shark here, as well as a number of hammerheads coming in to hunt stingrays in the lagoon).
bonus location:
Wolf Rock (Queensland): a few hours north of Brisbane (possible long weekend destination):
https://www.diveplanit.com/dive-destination/wolf-rock/(take a look at the video...)
Just had a friend dive this as an add on to his GBR trip - he said it was better than all of the GBR (except the Coral Sea outer reefs).
However, given your location, have you thought about heading north (instead of Northeast)? A trip to Bali and then to Raja Ampat is actually quite easy from anywhere in Australia (much easier than from Europe or the US). Amazing diving with the chance of diving with mantas and unequaled biodiversity of fish, critters, and coral.
Others have mentioned Palau (been several times - very enjoyable, recently dived it with my 15 year old daughter), Yap (mantas), Pohnpei (we had great dives with mantas, and more reef sharks in the passes than anywhere else we've been), and Tonga (snorkeling with humpbacks),
What we enjoyed most of living in Sydney (and traveling the region) was the opportunity to take long weekends to dive in different places. For example, we spent several long weekends diving Lord Howe Island - a really cool destination just a couple of hours from either Sydney or Brisbane, we also took a trip to Christmas Island (flying out of Perth) that was great for whale sharks.
Matt
Thanks A LOT Matt for your comprehensive advice, especially about the nannies in Fidji! When others were referring to great resorts for families in Fidji i had pictured just the Mariott-like swimming pool focuses places, but there are ways to travel there together with our kids & get them looked after while we do a few dives, that's heaven!!
In that case, i'll probably rule out Fidji from my short list for that particular parents-only trip, just because it means we can go there as family, this is great.
1/ Solomons really look interesting! When you're referring to snorkelling with reef sharks off the dock, you mean like by a pier, on the shore? Which place was that in Solomons?
That's the kind of photographer-friendly place i am looking for (already picturing those split-shots with sharks
).
2/ PNG - certainly one day, and same applies to places you've mentioned like Bali (we've been twice), that i'd consider very doable from south east Asia or even Europe. Here i want to shoot for what's hard to touch when going back to good old Europe.
3/ French polynesia really looks like a good match for shark action (without feeding). I am picturing passes with lots of current (never dived one), is it still manageable to get close to these sharks?
Were you saying Pohnpei was better than French Polynesia for sharks?
Looks like you've really made the most of your time in Sydney, I envy you and can't wait to take my kids diving when they get of age!
How does Lord Howe island compare to the above for approaching sharks/big stuff? Is it shore-based diving or boat?
When it comes to long weekends, i'll tend to favor places accessible by car (e.g. South West Rocks) just because we both dive rebreathers, if we're to bring kids along too, the airfrare becomes a killer... that's unless Lord Howe island really has something special.
However i've just watched the video on Wolf Rock... thanks for the tip, it looks amazing, i am seriously considering going there 
We could also have that discussion in french language, relocated on the other board you know.
My experience in South Pacific (Disclaimer : Vanuatu is where I grew up btw, kantri blong mifala) :
Fiji , absolutely lovely islands and people. You are a social freak if you don't shed a tear when they usually sing the "Isa Lei" on your departure. Diving wise, it's color paradise I don't remember a dive in the wild nearby Vatu I Ra or Bligh Waters where I didn't spot a reef sharks and large pelagics such as tunas or spanish mackerels.
Even people who say they are opposed to shark baiting/feeding are impressed with B.A.D. (Beqa Adventure diving) operation where tigers and bull sharks are regulars.
Ovalau and Wakaya are supposed to host hammerheads too.
PNG : Only went to Milne bay (Tawali) but I support the idea that it's probably the place where corals are the liveliest. Sometimes hammerheads are seen bit not frequently.
New Caledonia : lots of manta rays even on day trips from Noumea, some sharks and sometimes silvertips (Dumbea pass). Expensive and not very tourist friendly, but easier for french speaking tourists, i wonder how exclusive english speakers can cope with NC (although it was once a prime destination for Japanese marriages).
Vanuatu : Forget the diving for a while and enjoy Vanuatu on land, best country in the South Pacific for topside attractions (tribes, culture, volcanoes) or... food (perhaps some of the best beef in the Pacific and also best snack in the world called Tuluk). only the Pres.Coolidge is worth the diving (Impressive even for the non wreck divers as I am).
French Polynesia... yeah, lots of sharkeys and rays, less biodiversity... uber expensive. Not that attractive to me since I was once stranded a month in Papeete maybe.
Hi Luko,
Actually i write quicker in English than in French, and i thought i'd dive in the Wetpixel pool of knowledge when it comes to South Pacific diving.
In the French forum i see lots of people diving South East Asia, Red Sea and Carribean, whereas it seems to me that Wetpixelers go further East (or West, depending on where you are). Anycase, i'll definitely ask that community for French Polynesia specifics, as i know there are experts there.
Back to this topic, lucky you for growing up in Vanuatu!!
I am interested to know about B.A.D: in the way they operate, what would you say impresses people who are against shark feeding?
On Vanuatu and beef: i was today barbecueing next to a family from Vanuatu -> super friendly, we've exchanged food and beers (and they did plan to BBQ a lot!). I see here lots of people planning dive vacation in Vanuatu, i guess it's only around the President Coolidge then, and i wouldn't mind focusing on 1 wreck for creative photo projects, but i am wondering how crowded it gets? are there times of the day when its gets less busy?
Maybe your Papeete experience is why you're not keen on French Polynesia - all the divers i know who've been there were soooo happy.
@all, if i may continue asking i'd like to complement my research with 2 questions:
1/ still in south pacific, can you advise any place where sharks/big stuff (rays, turtles, groupers...) can be seen in shore dives / unlimited diving?
There's nothing we love more than a good shore dive, especially when we can decide by ourselves how long we stay (we dive rebreathers, so in Bali we did 3+ hours dives and loved it, but it missed big approachable animals).
2/ for those who dived Australia, on the criteria i've noted before i'd like to know how those places i've noted rank: Lord Howe island, North Solitary Islands, Byron bay, Yongala wreck (Townsville), Ningaloo reef (+ Wolf Rock where i already have information from Matt
).
again, thanks a lot for your advices
cheers
Nicolas