Self isolation of people (with unknown status or no symptoms) has not been very successful (up to 30% were not at home), I expect that is because they take it less seriously. I also expect actually getting a positive result would give most a reality check, and they would then be happy to correctly self isolate.
Are you sure they are not the figures from Victoria for those who had tested positive but had not met their self-isolation obligations? As most if not all (non-celebrity or sportspeople) international arrivals in most states have had to quarantine in hotels, I would suggest your latter suggestion that 30% hadn't met their self-isolation requirements, comes from the very same group you suggest would correctly self-isolate knowing they're positive. You don't think those becoming desperate to return home wouldn't be the most likely not to jeopardise their quarantine and risk hefty fines if they didn't follow the quarantine rules?
Just some consistency both within the States and federally would be nice. At the moment all we have is a totally inconsistent set of rules that are driven by political expediency and a desire not to lose face by admitting that some earlier decisions had been a little over the top.