I moved here 6 years ago and there are lots of pluses. Friendly, decent weather (much better than Manchester anyway!), generally quite safe (of course there is crime but I feel much safer than in UK), outdoor lifestyle. Lots of things to do that are cheap/free e.g. beaches, national parks. Slower pace of life (which can be good or bad depending on your point of view!). Also we can afford to pay our mortgage and live on one income which is good as I am soon to become a SAHM.
The biggest pitfall is being so far from family, how close are you to yours? I keep in touch with mine a lot but it will be expensive to fly back regularly to see them, not to mention it's a long flight with small kids. It's a great place to bring up kids though and there seem to be lots of Brits emigrating here at mo.
My dh runs an IT recruitment company here in Auckland and there seem to be quite big shortages of skilled IT professionals. He seems to be constantly placing UK expats in IT jobs! Immigration would not be a problem if your dh is in IT. There is a website www.workingin.co.nz which has details of companies and recruitment agencies. Dh's company is on there - CPU Recruitment - or I can always pass you his email if you or dh want to email him about job opportunities here.
Salary would prob not be as high here as UK but still high by NZ standards.
I don't know anhyone who has regretted moving here, though obviously most people have their moments of homesickness. Oh, the shops in the UK are much better, but I'm used to that now!
At the end of the day, if you really didn't like it after a year or two, there is nothing to stop you moving back to UK (though I understand you may not want to move kids around too much).
Good luck with whatever decision you make.