and so it's natural that there is a high level of permanent migration as the 'fear' of living overseas has been conquered at an early stage, they've built friendships/relationships and so don't come back from the OE, etc etc.
or it could be that the standard of living is so much better where they are that when they do visit NZ, they realise they would be better off not coming back to live.
The other aspect of migration is that we need to determine which % of those are 'NEW' migrants from places like Asia that know their standard of living would be far better in NZ than where they were from.