I'm a NZer who's lives in the UK. We were home last month visiting my family. For me, Christchurch is the only place I'd live if I went back. It's smallish (pop 400k), has lots of facilities, close to beaches, an hour from mountains etc and is just beautiful. I am envious of friends who go to work and then go windsurf or for a walk on the beach after work. Work is just a part of the day, not the bulk of it. We visited family in Auckland and I really didn't like it. It's seems to be just another big city (albeit a very beautiful one) but has traffic problems, felt crowded etc to me. I guess it depends what you want.
We found the cost of living to be more expensive than on previous times we've been home, but still overall cheaper than the UK. Certainly food shopping and eating out was comparable to here, although the quality and variety of food was better and greater. Petrol is cheaper, as are cars.
I was amazed at the number of British voices I heard while there. In my Mum's v small town (around 1000 residents) five new English families had moved there in the past month. You won't be short of friends from home!
For more info re wages etc, contact NZ House, Haymarket, London - they have an information service and I'm sure can help.
Re health service, have recent experience of hospital when DH broke his shoulder and collarbone. Very impressed with doctors, nurses and other staff there. They all seemed very relaxed, happy and friendly. I'm sure it's not a perfect system but didn't come across anyone who seemed miserable! Technology seemed to be very good too. Also went to GP with various ailments with kids. GPs too seemed very happy and had time to chat. You pay to see a Doctor there and most people have some sort of health insurance. Emergency treatment is free.
I'd go for it if I were you - if the opportunity is there, grab it with both hands! Good luck.