We did exactly this late last year.
In town check in at 1030 ish. You'll need to buy a ticket for each passenger to access the check-in zone at the station. Drop bags but no need to travel on the train.
Back to the hotel for a late checkout then to the airport with hand luggage only.
Lots of OW lounges to check out. Pier and the Wing are both amazing with the pier having a dining experience.
You can lounge crawl, collect stamps on your boarding cards and still make it back to the Qantas lounge before take off.
Enjoy!