Simon, you lucky b@stard!
God I'm green.
As for your questions, there is a pretty active breather scene there, with gatherings for diving, pub and pizza.
As for sorb, it's available, but no nuffin about the price.
The tanks, that's a snakepit. I never had any problems getting OS tanks tested and therefore approved and usuable, but that was in Cairns, QLD.
But it seems that virtually everyone else has problems with them....
A possibility would be to contact bazza at southern cross divers in Sydney,
or ofcourse DIVEDIVEDIVE (D3) in Brisbane. They do quite a lot with inspo's and should be able to help you either "clear" your tanks or source replacements.
Enjoy living in Australia!
denz.