10 hour transit + flight/disembarking/luggage/immigration etc? That's minimum 12 hours.
You can drive MEL-ADL in 8 hrs.
But to the original question, leaving the airport shouldn't be an issue for an Aus citizen.
Getting back on the MEL - ADL flight, not sure why you couldn't go back through immigration/security etc (with or without a 'D" sticker) the next morning with a valid boarding pass and passport. Luggage should be checked through so as long as she has some hand luggage with the necessities to take to the hotel, I can't really see it being an issue. Probably will get asked some questions, but no "laws" are being broken that I can see.
From the Malborne Airport FAQ:
I am a passenger in transit at Melbourne Airport with a long wait until my outbound flight departs – can I leave the airport and return to catch my outbound flight?
It may be possible to leave Melbourne Airport and return for a connecting flight, depending on your circumstances. It is the travellers responsibility to ensure all potential requirements are met in order to board a connecting flight.
These include but are limited to:
- meeting all security, biosecurity, customs and border protection requirements for entering and exiting Australian borders (e.g. appropriate passport, visa, outbound flight documentation as required)
- satisfying all specific requirements of the airline providing the connecting flight
- ensuring sufficient time to return to Melbourne Airport and complete all steps in order to board your flight on time, including check-in, Customs and security screening
- ensuring appropriate arrangements are made for any luggage.
www.melbourneairport.com.au