Paul1977: In regards to the original question, can't the replacements be marked as such "Replacement goods - no GST payable", or something to that effect?
GST is calculated on what you pay for the goods, and you haven't paid anything for the replacement items.
EDIT: Men's clothing is subject to 10% customs too, so sucks if they make you pay that twice too. And don't forget the import entry fee, or the biosecurity fee.
BUT... won't the sender still list a value the second time around, which they will have to for customs and maybe insurance purposes (or will they list it as zero value?), Customs are going to see that and hit you up again.