The fact that most people live off (and have done for a couple of decades or more) is why there are no longer ‘thousands’ of empty rooms on bases. Many of those facilities have either been demolished or are not in a state where they can be easily converted to quarantine facilities. Think shared ablution facilities (not a great thing when trying to quarantine people), difficulties arranging for meals to be delivered to rooms, transport and logistics issues transporting people to/from capital city airports, lack of suitable mass laundry facilities, security issues on the ADF bases, etc.
Some bases may be able to handle some people but on aggregate across the nation not on the scale you’re suggesting.
I presume they’re doing their day jobs?
I don't know of one accommodation block that has been demolished on any active base in the last 20 years.
As for infantry and artillery doing their day jobs, are we off fighting any conflicts at the moment? No, most military people are training all the time which is what they do when not overseas. As none of our military (apart from navy) is going anywhere anytime soon, maybe they can be utilized somewhere else?
I will admit most military accommodation is pretty basic but hardly any still have shared bathrooms and wouldn't require much to get up to standard.
Friends of mine who finally got home after having numerous flights cancelled, said they would have done quarantine in a tent if that was what's required.