I've opted out on principle. There have been so many IT stuff-ups in this country (let's start with the census). Apart from that, even the federal health minister doesn't know how it's going to operate. Yes, (contrary to his beliefs) the police can demand access to it without a warrant, it seems. Is it accessible if you're overseas? They don't know yet. So many unanswered questions.
At my age, I care little who knows what about my health history; but for young people, it could be used against them for employment reasons, if it's not secure.