It's a number 125 o-ring (3/32"w X 1 5/16"id X 1 1/2"od). Make sure you get a Viton o-ring. The cheaper nitrile (or buna) rubber ones the hardware stores sell can't take the high temperature and will break after a few runs. I usually put on two viton ones for extra insurance.

I have some extras left over that I'll never use. I don't know what postage to Australia would cost but if you reimburse me I can mail you a couple.

Joe