At the moment, probably just joint military exercises.
I don’t know the process, but a host country can invite US military personal equipment to do joint exercises as a precaution.
I think what we need to watch for is a military build-up in Australia. That would mean something.
Throughout the Cold War SAC had Darwin as an alternate B52 launch and recovery site (there’s a B52 in the Darwin Aviation Museum today, next time I’m in Darwin I’ll have to visit).
Just about everything in the US inventory has been here for various exercises for decades. What is happening is that back when Obama was President he and the then Aussie PM agreed to Australia increasing its hosting of US forces including building new facilities to accommodate them.
For example we’ve basically had a permanent Marine battalion rotating through Darwin for years. They use our Bradshaw Military Training Area near Darwin which is about 3400 square miles in area.
This latest thing is that the Australian Air Force base near Darwin is being modified to be able to support 6 B52’s including maintenance, fuel, and weapons storage. This complicates things for the Chinese as Darwin is harder for them to hit (conventionally) than Guam or Okinawa and gives an additional southern vector other than Diego Garcia.