In the 2022 Australian election campaign, the Australian Liberal Party has proposed an "Autonomous undersea warfare capability for Australia's navy" as part of its electoral platform (5 May 2022). The Liberal Party lost the election, but there is merit in the new government implementing this idea. Rather than a purely autonomous craft, I suggest optionally crewed submarines, with the ability to carry about seven people (three crew and four special forces passengers). Australia could acquire a dozen such submarines and two submarine tenders to support them, for the cost of one Collins size boat. This would also ease staffing, as the submarines would require far fewer crew.
The proposed submarines would be eXtra Large Autonomous Underwater Uninhabited Vehicles (XLAUVs), designed to operate in Australia's large maritime region. These could be developed using an engineering approach similar to that of the SpaceX Dragon spacecraft. SpaceX took the approach of first proving an uncrewed cargo version of their spacecraft, before adding manual controls and seats. With a submarine this would allow for more rapid development and operational use.
Such a small craft could be used for the same primary roles as a full size submarine: surveillance, deployment of mines, and delivery of special forces. They would not be carry large conventional torpedoes, for a direct attack. However, they could deploy self-propelled smart mines in this role, with the submarine leaving the vicinity before the attack, to increase its chances of survival.
Pakistan Navy, CC BY-SA 4.0, 2018
XLAUVs and mini-submarines have a shorter range than conventional and nuclear submarines. The range can be extended by the use of submarine tenders, to refuel, rearm, and exchange crews, closer to the area of operation. The tenders would be able to be designed and made in Australia, derived from Australian stealthy warship and fast military transport ship designs developed for the US military.