I've got a similar question.
I'm flying in J from on BNE-PER on QF937, to connect with QF9. It departs at 1300 hrs. I have a connection to a BA flight at the other end, so MMB is already saying I can't check in online. How early can I check in and drop off my luggage? I don't know if they're still experiencing delays at the airport that were reported a while back. But to allow for that possibility and also to avoid being caught in a major traffic event, which does periodically cause huge delays on the Gateway Motorway, I want to get to the airport quite early, check in and then just wait in the lounge.