Not sure if this is well known in NZ, but most goods are cheaper already in Aus so you can get fairly good deals on a lot of goods if you happen to be visiting family over the silly season with GST refunded back to you.
Terms and conditions here: https://www.abf.gov.au/entering-and-leaving-australia/touris…
It can be a good deal, but you also run the gauntlet on the queue each time. Some days you walk straight to the front, and others will be a 1hr+ wait. So you have to hope for the best.
The process is fairly easy. Show them the goods you’re taking out, the receipt confirming purchase, and they hand over the $$ (can’t remember if card or cash).
I’ve done it several times (I’m there every month), but sometimes Sydney airport (particularly), is a mess and there’s no time.