If you end up in Wellington there are lots of great clubs. The Wellington Marathon Clinic have 2 hour pack runs every Sunday morning and are really friendly and not competitive if you are looking for a more laid back club.
There's definately not a 10k or half mara on per month in Wellington, The races here are good though and there's a weekly 5k series from around August until April run by Scottish Harriers which is really good.
I lived in Christchurch for a short time and didn't do lots of races but did a few run by the Sri Chimnoy Marathon Team which were really good. They were 10k and you got a really delicious breakfast afterwards so i'd recommend that!