On NZ trips to the Poor Knights (a real macro heaven), accessible from Tutukaka on the North Island, in Nov '00 and Oct '01, botttom temps were 16-18degC. Most locals donned a 7mm, as i did. DMs generally wore a 7mm when working versus a drysuit while off on their own. I can highly reccommend Dive Tutukaka for their flexibility, topside service and thorough knowledge. Most of their Kiwi DMs were marine biologists and were willing to discuss their research or my interests with equal enthusiasm. They have a number of international DMs on staff as well. Temperatures can vary substanially from year to year, as the australasian current dips south occasionally, depositing in the Poor Knights various planktonic forms from the tropics, where many are tolerant ehough to grow and trive, but not reproduce.
Incidentally, I bought my 7mm titanium-fleck semidry Apollo in Auckland, during my Nov '00 trip. At that time the exchange was 1US=2.4NZ (a ten year low for NZ), which made it a good purchase to begin with, but it was also on sale ... and ... the shop voluntarily tossed in a full week's rental of two tanks/dive computer/weights/BC/regs/....! In the past year, however, the NZ and AU dollars have gained considerable strength against the US dollar (see the history by googling for FXGraph). Last week, the exchange finally dipped below 1:2, but this still makes for great vacation at a superb value.
I just came back from two November weeks in New Caledonia, which might qualify as coral sea diving - certainly spectacular below, and quite enjoyable topside if you enjoy speaking french. A recent article in the journal "Science" (295:1280-1284) on marine biodiversity gives NC world ranking of 7th and 11th in terms of biodiversity and - unfortunately - threatened endemism, respectively. Dives there were 23-25degC, which I dove using a new 3/2mm w/hood/gloves/booties. Locals, wearing 5mm., were saying that normal temps for Nov/Dec are 27 and up.
There are about 15 different dive operations in NC, most on the big island. I dove with four and found service to be highly variable. With some, it was rush-rush-rush to get to the site, then young DMs who swam way to fast, and led short dives (e.g. 38min). Others were superb, including Dennis who runs a zodiac on Ouvea, and Anabella and Tawake who have the 53' ketch Harmony, which they keep moored off of Kuendu Beach, which is located 10-15 minutes north of the downtown area of the rat race called Noumea.
For reference, here in SD, bottom temps are generally 9-12degC, which i dive using a 7mm mostly-dry, with two 3mm hoods, thin gloves and thick booties.