I think they are buying 70 of the SEP3 versions, which are the best made. No word on what will happen with the other M1's, but I can't see them being retired.
I believe Australia still has a very robust rail system, so these tanks can be deployed rapidly where they are needed. If they have the truck transporters then they can go anywhere there are roads.
In any event, just 15 or 20 of these beasts ending up in an enemy area of operations would cause a lot of issues for the invader.
These are very few Abrams tanks made new these days. The plant is all about remanufacturing them. The ones Australia bought 15 years ago came off the same remanufacturing production line as US ones. The only way you could visually tell them apart was the little red Kangaroo painted on the side. They probably lacked the DU armour package for Australian conditions, and maybe one or two comms systems which are not used by Australia.
The 'new' ones will be remanufactured just like the US Army ones, but possibly again without the DU armour package, etc.
The 'old' Aussie ones will just go into the start of the remanufacturing cycle.
Australia has truck transporters for the tanks, and they fit the amphibious assault ships they have. Also the C17s that they have can carry the Abrams.
Yes, a few on a deep cavalry raid will do terrible things to your logistics chain. They may even last long enough to get home if they have ground based air defence that can keep up with them, hostile air is their single greatest enemy (followed closely by friendly air).