I think you need to really look at the lifestyle in NZ. It is a beautiful, amazing and wonderful country. I couldn't recommend enough that people go and visit it and experience the wonderful place and lovely people for a holiday.
But living there is another story. It is so insular, isolated, expensive and nothing ever happens, traffic is terrible and public transport non-existent which makes many things we take for granted in HK impossible to achieve due to traffic (Assuming you live in Auckland). People are not looking for new friends either, they are still friends with the kids they went to primary school and secondary school with, they are nice to you but not very inclusive if that makes sense.
For the right person, at the right stage in their life, NZ is paradise. For most people it is a very easy place to romanticise and very difficult place to migrate to. I would suggest you make sure you are not looking at it through rose coloured glasses. (If you are the right person then go and you will love it!).
As a parent you should also know that teenage suicide in New Zealand is the highest in the world by an unbelievable amount (and has been for a very long time) it is a really serious problem that they can't seem to resolve. Crime is also much higher than in Hong Kong (but then I think crime is in most places). I would suggest you read the Herald online each day for 6 months and see if the country in your mind matches with the daily reality.
NZ is very expensive to live in. Rent is cheap (compared to HK) and everything else is expensive. Especially on NZ incomes. Buildings have poor insulation and construction, heating costs in winter are substantial.
Where will you live? Auckland? What suburb? Have you looked at property prices? Will you rent or buy?
What schools will your kids go to? (good public school districts are expensive real estate and private schools are pricey). Bad schools are BAD!
Car (or 2 if you have a partner - which will turn to 4 once your kids turn 15 and can drive) is the norm as there is no public transport.
Food is expensive, cell phone plans are expensive etc etc Travel (because NZ is so far away from anywhere is prohibitively expensive for most people to do more than an annual trip - that is the upper middle class).
ACC Levy is 1.4% or something on top of tax (not sure actual figure but google can tell you). and GST is quite a lot now, maybe 12-15% I think - again google as that I'm really not sure on).
(I am biased, for transparency, I would never live there again. 8 years was more than enough for me. Love it, wonderful place, but just not for me personally).
Whatever you decide I hope it is the right decision for you!