Sidestep: I'm sure this imbalance - your $400 import duty/GST exemption - will disappear sometime in the future, becoming more like say, Canada, where with the addition of GST (on every purchase) & shipping, the US price advantage over buying locally is reduced.
There's no exemption. It is just that the costs associated with collecting GST are higher than the $60 collected. For example if the sender doesn't collect GST (and how small stores would do it?) then Customs will need storage space for all goods while waiting for payment. People to move the goods from the pallets to storage, people to chase payment, people to process payment, people to move out of storage, trucks to move to NZ Post or Couriers, etc.
All this costs money and $60 is the minimum where they can do it.
Start charging for everything and you have a government agency that will need a super budget, more employees and so on...