And those four ships were built in the very early 80s so it's not as though they're some decrepit WW2 veterans that sat in reserve for decades. They were still operating into the 2000s.
River Embly was scrapped in 2012 and River Boyne followed in 2014.
I believe the other two went to Singapore sometime between 2012 and 2015 to await disposal.
I'm inclined to believe the Australian government would use these four, if they were still usable, for cargo runs around the Australian coast. I don't think they would be willing to risk losing them for resupply of overseas forces unless they could provide them with sufficient protection - they're just too valuable for helping re-establish transport within Australia.
Edit:
DOH!
Meant to add the following information for River Embly & River Boyne - taken from
https://flotilla-australia.com/3anl.htm : -
Specs - 51,035 gross tons, 76293 dwt. Lb: 255 x 35.4 metres. 16 knots. Single screw, turbine powered, twin boiler design using coal as fuel, fully automatic with UMS certificate. Speed 16 knots. Crew 38.