So, sorry, but "traders here want $6 or more" is frankly insulting to traders, it's NZ Post and other couriers who want to charge $6 or more to deliver to your rural address.
As somebody else indicated, the reason that international packages can get to your door out in the wop wops for essentially free is down to the way international mail works, while I understand there have been recent adjustments made to the way it's charged by the UPU, I think you will find that in the case of a large volume imbalance (lots of small parcels incoming, not many outgoing) it's the receiving postal carrier that gets landed with the majority of the costs and they are unable to pass that on to you.
Sorry to be insulting to traders but the point I am making is how it looks to me as a rural customer, not how it got that way or whose fault it is. If I buy a dohicky on TradeMe, it costs around $6 (or more) just to have it sent to me. If I buy it on Aliexpress, my total cost is probably a third or less of that on TradeMe. It is a no-brainer.
Those traders can't be responsible for their direct costs to ship something to you being much higher than those of a China based seller. The issue here is that you didn't understand the difference between their costs (which you now do, presumably)...
So either you expect traders to make a loss shipping you stuff (completely unreasonable) or you're just venting that you believe the entire transport industry in NZ is overpriced (unlikely). In either case, blaming Trademe sellers for charging what it costs to send stuff to you is silly.
Cheers - N