There is lot of information out there, but few things aren't clear.
So if overseas retailer isn't registered for NZ GST and selling items for eg. NZ$750 without GST. What happens now when it reaches Customs. Will we be charged Custom Duty as before? or it is great opportunity, to buy things which are under $1000 till they register for GST.
Yup. No gst/custom charges if less than $1000 incl shipping and not registered.
The thing that might trip you up, if you pay less than $1k and they deem the item is worth more than $1k then I'm not sure what happens, I assume you get charged gf st