There's some flexibility if you show up at the airport within a few hours, the "flat tire rule" which is the norm in North American airlines (but not European airlines). Not sure how it would apply to an AP booking in Australia though. But if your first flight is cancelled and you show up the next day without having dealt with it at the time then I would expect your booking will be gone already.

Some airlines are also issuing blanket travel waivers, AC has something in place at YYZ already. It's possible if there's widespread chaos at SYD AC might issue some waiver for bookings out of SYD but I don't think there's been any talk of anythinglike that. But it would still almost certainly require making changes *before* departure. And AP bookings may still require going through AP to make changes.