Chiclanagir

I will be coming out to Christchurch in about 18 months as my daughter is marrying a Kiwi. Obviously I want to have a holiday as well and would have liked to have visited the Whit Sunday Islands in Oz after the wedding and before coming home to the UK. Is that sensible as it seems to be going backwards! What about Snorkelling in New Zealand or is the water too cold.

asiatic

Probably best to buy/rent a wetsuit
Take a look at these guys >>> http://www.blackcat.co.nz/

annaerb

Whitsunday Islands are great, we stayed at Daydream Island and Hamilton Island, Haven't been diving though so can't help there.

lapsed kiwi

If you get a "round the world" ticket, you could fly out via LA, then return by Sydney, Cairns, Singapore. You could also incorporate more than 1 NZ airport if you want to see NZ, eg arrive AKL leave CHC. It shouldn't cost much more than a straight return, last time I did this it was about £800. The other amazing option for snorkelling and scuba is Rarotonga, and if you fly Air NZ you should be able to get a stopover there at no cost.
If you have a light wetsuit I'd bring it for NZ, the water is warmer than UK in the summer but still not exactly tropical.

newkiwi

There are swim-with-the-dolphin type excursions available for NZ, i think near kaikoura, but a google search should point you in the right direction. You would need a wetsuit, but these are easy to hire, and if you sign up for any type of organised excursion the wetsuit hire will be included.

You should be able to fly back via aus easily enough, and take in a visit to the Whitsundays

Chiclanagir

This sounds like the best idea, wedding first and then the holiday. Loved the whitsundays, but, where is Rarotonga? When we last visited Oz we came back via Fiji but I was disappointed in where we were which was really stoney and so dirt poor.