I very much doubt they will be American Subs. the American SSN's , seawolf , Virginia , LA class are all way to big with way to large a crew requirement. Far more likely to be a Astute Class Hull/ drive train / reactor with mostly American weapons, fire control and sensors. would lose the VLS tubes but that seems a reasonable compromise.
Also expect the Large amount of the subs construction to be done in OZ. American sub yards are full , UK's isn't as they are coming to the end of build for Astute, So there is an ideal opportunity to build the first hull in the UK while skilling up AUS engineers while at the same time building out the naval base assets needed to support SSN's at Australia's two main fleet bases. On top of that it would bridge a large amount of the time gap between the last astute and the next gen gen of UK SSN being built, not losing skills and workforce is very important.
If this really is a Force projection to the China Sea. US/AUS probably want to be able to station subs in Darwin as well.
In tems of the now cancelled "Attack" class sub, so far its been 5 years and 2 billion dollars and not much to show for it. in all honesty the wrong bid one that tender , mostly because the Japanese completely fucked it up.
This video here does a great job of summing up ( over a year ago) what a basket case the Attack class project was.