Yes, the frame fits all, so you can just buy the cushion.
I think small mask comes with a slightly smaller headgear but its not a big difference.
Supplier #1 sells the whole set for US$99, cushion by itself is ~US$33.
Can't remember how much the delivery was to Down Under but they did have a "flat rate" so it may be worth "filling" the box with other goodies.
I got a spare set so that when I wash the headgear I have a new one ready to use and the washed one can dry properly.