over here in the US, each state has a registry where you can register to become an organ, eye & tissue donor. some places here let you do it completely online and then send you a card of some kind that indicates you are a donor, so the coroner knows where to deliver you first.
I am not familiar with Australia but it seems very similar,
you can just register online here. it will ask for your medicare info. I know it says "organ" specifically, but according to health.gov.au
this covers all forms of donation, including tissue.
I would say just register regardless and get your donor card. in the event of your death they will look, decide what what they can or can't salvage, and then proceed from there. the worst thing that can happen is nothing, but that's not a failure on your part.