The answer has to be off the shelf - and even second hand. The strain of staffing and operating a new class of submarine will stretch Australia a long way (see the Sir Humphrey article referenced above)- adding in building and (even worse) a new or modified design would be insane.
The Uk has enough problems getting Astutes out of the door from a single yard in time to get the new SSBN's on track (and thats after 60 years of building SSN's) - any new production has to be in the USA - give a decent order the yards there are in a better state to up production over 3-5 years - which is how long it will take to train the crew (s)
Building them in Adelaide - a very long way away