Buying a Nuclear Shortfin might end up being easier and better than trying to convert it to a diesel-electric sub.
I think "Shortfin" refers to the conventional design. The nuclear version is just "Barracuda".
Pedantism aside, one must wonder about our politicians. How did they manage to make conventional submarines more expensive than SSNs? If we are going spend so much money, maybe we should be getting SSNs to make it worth the money. However, were it up to me, I would have gone the Japanese subs, built in Japan, lower risk, available on time, they already have US combat systems and we would save money. In terms of economics, we sell resources to Japan, so it would still return at least some money to the Australian economy.
I wonder if this Shortfin project is the most sensible idea? Defence resources are limited for smaller powers like Australia and we do need to be careful how we use those resources. There are lots of other things we could do with the money 12 of the world's most expensive conventional will cost. Like get more aircraft. Or increase readiness levels. Quite possible both with that much money.
Historically submarines proved very dangerous. I can't think of any battleships sinking aircraft carriers, but quite a number of carriers were sunk by subs. Yet submariners typically suffered the highest causality rates in their respective navies. It is true that modern submarines haven't really been tested in sustained war. So nobody really knows how they would fair today against state of the art ASW. I rather suspect that the balance hasn't changed much and that subs will continue to prove dangerous but they will also suffer high attrition.
For Australia, we need subs to preform covert reconnaissance and minelaying. Going after enemy subs or surface warships is, in my view, too risky. Trading a sub for a destroyer isn't worth it. Though having a crack at a carrier would be worth it, though the sub might not escape. Aircraft using advance cruise missiles, torpedoes and sea mines are a much better way to fight naval war. Especially the sea mines.
Maybe we would be better off getting good at finding and sinking subs rather than responding to increasing numbers of submarines in regional navies by getting more and bigger subs of our own? So Poseidon aircraft look good to me. If we had to get more of something...
I recommend looking at the Japanese campaign against Australian coastal waters. The Japanese didn't achieve much because they didn't commit much, and the Australian defence didn't achieve much due to lack of detection equipment and possibly experience. What it showed was that finding small numbers of submarines in vast areas of ocean was problematic. Quite a contrast to the density of the Atlantic battle. We need good situational awareness underwater as well as on the surface and in the airspace in our region. Some sort of broad area detection system like the Americans had in the Cold War. Plus sufficent Poseidon aircraft. And sea mines. Sea mines are a very efficient way to balance disparities in sea power.
We will need to replace the Collins class but I fear the project has been hyjacked by vested interests and money will be unavailable for other needed defence purposes.