They’d be better off buying Virginia class boats. Nuclear subs have the range Australia needs.
The Australian navy had struggled to retain personnel on its existing Collins class submarines, which had crew complement of about 50 personnel. The Soryus have a complement of about the same size. The Virginias in the USN need about 120 personnel. The RAN will need to ramp its budget significantly to train those many personnel.
Then there’s the issue of maintenance costs, repair and overhaul facilities in Australia, which are most likely not meant for larger nuclear vessels.