Deisel/ electric submarines need to surface for air every few days.
One of the supposed advantages of nuclear submarines is they can stay submerged for weeks or months without needing to surface for air.
The Gotland class of submarines from the Kockums shipyard in Sweden are the first to be powered by sterling engines while submerged. It allows them to stay submerged for weeks at a time, and they are also very quiet, stealthy and highly maneuverable. In trials, they have recorded significant kills.
What criteria were used to select nuclear-powered submarines for Australia? They are an extremely poor choice on cost and times scale to deliver. It would take decades to build up the infrastructure and expertise to be able to manage and handle nuclear-powered vessels. The political issues with nuclear power alone are a major barrier to a successful program.
Australia currently has a fleet of six Collins-class submarines. The Gotland class of submarines would be worth exploring, as they were both based on designs by the Kockums shipyards in Sweden and the transfer of technology would be more seamless, compared to nuclear-powered submarines. With sterling engines they may offer many of the advantages that the nuclear-powered submarines are supposed to offer.
The Collins class is an enlarged version of the Kockums Västergötland-class submarine.https://en.wikipedia.org/wiki/Collins-class_submarine