The regulations on paying GST on 'low value' imported goods changed 2 years ago and the current regulations can be found
HERE.
Essentially it raised the threshold from NZD400 to NZD1,000 and also mandated larger sellers including online selling platforms such as eBay to register for NZ GST and add (and collect) that tax at time of sale. The exception is if the seller sells less than NZD60,000/year to NZ customers which is the same threshold that NZ businesses' turnover is before having to GST register.
This brings us into line with the Australian GST regulations and their equivalent is a AUD75,000 threshold of goods sent there. We have a number of Aussie sales but don't sell anywhere near AUD75K of good there so don't have to register. Therefore our Aussie customers aren't charged AU GST (or NZ GST as our checkout deducts this for overseas delivery addresses).
For NZ buyers the way to avoid paying GST on low value imports is to buy from smaller traders (directly, not through eBay, etc) and keep your order below NZD1,000. Splitting a larger order into smaller consignments doesn't work as Customs do monitor recipients' activities over time and will combine multiple consignments and charge taxes accordingly.
The incoming shipment I mentioned above is a trade sized order of stock for us and its value is way beyond NZD1,000. We use a Customs and shipping agent to manage the transport and importation formalities which are far more complex than for 'low value' retail sized orders like yours.