IIRC regular domestic stamps can be used on international post but they will cost more (international stamps don't include GST but domestic stamps include it as part of the cost).

I haven't used stamps on international items before but you could probably use a $7.50 international stamp and plonk an old stock domestic $1 stamp on there to make up the rest - it's not like the $1 stamps are really useful for much else these days. A $1 domestic stamp should technically be worth 90c + 10c GST. Of course though, just ask the post office if you're unsure whether a $1 stamp (or $1.10 if need be) can be used.