Well ive just had first hand experience of this having had a phone call yesterday to pay the fees.
I normally try to keep my purchases under $1000 to avoid the GST and import tax ( extra 5%) but I figure that seeing as everything is now subject to GST, you might as well buy bigger to avoid multiple purchases.
So my normal AU$800 turned into over AU$2000.
Yesterday I received a text message and then a phone call from the carriers FedEx NSW.
Taxes are to be paid PRIOR to entry to the country and customs approval and it appears to be subject to a AU$20 admin fee..other carriers may charge more or less.
But, if you import from a non GST registered overseas company, then expect a phone call at any time and have the money ready on your credit card.
A $20 fee on a $10 item is painful... $20 on a $2000 purchase, I can live with.