While I completely agree that Australia should not forgo the SSN as they give us persistance and a significantly lower indesretaion ratio. Having an SSN sitting off a choke point is going to cause an incoming task group a lot of angst notng those choke poings are all in someone elses water. Unless the threat declares war on all these coastal states there will limitations on what they can do.
However, on the SSK (Oberons were called SSG's for a bit by the submarine commuity) issue a Collins is still going to be a hard nut if it is in the right place. The Collins has a modified version of the AN/BYG-1 used in the Virginia (noting this is intended to be fitted in what ever SSN we build) and is undergoing a SONAR upgrade that will provide systems with similar capability as the Astute SSN.
Collins Class Submarine Replacement | DST (defence.gov.au)
Collins Class submarines to receive sonar upgrades | Defence Ministers
The vessel carries the same Mk48 CBASS as the US submarines and can fire Harpoon and should be able to fire Tomahawk (once purchased). It can alos carry mines which can be quite useful in making life dificult for a task group transiting a choke point..
The boat is also very quiet (the boat did have issues to begin with but it is a quiet boat .... irrespective of some of the harping by some of the ill informed). So if it is on station with a full battery it should have the similar cability as the future SSN to get to a firing solution. What cannot do is transit as quickly as an SSN and is power limited meaning once you have engaged you may not be able to pursue your target due to the need to avoid detection and the difference in speed (and its impact on battery life). In addition I would suggest no right minded CO would be firing harpoon in any situation where a plume of steam and smoke will tell the opposition where you are, however, the Mk 48 is lethal and long ranged and will make ASW defence difficult .
I would expect that the Collins would not be playing solo if a task group was moving through Lombok or similar openings. I would suggest a joint action involving Submarines and aircraft from the mainland would make life quite complicated for a task force deploying out of a choke point.