Aukus deal. Was Australia called in to share ballooning development costs?

Question

The Aukus deal is a trilateral security pact between Australia, the Uk and the US. The biggest part of the pact is the acquisition of nuclear powered submarines jointly developed by the three countries, although Australia will have a minor role.

The deal itself came as a surprise because initially Australia was not interested in nuclear powered submarines and to carry on the deal they had to cancel a deal with France to acquire diesel-electric submarines.

The huge costs involved in the deal:

Australia's PM said the plan - which will cost Canberra up to A$368bn (£201bn) over 30 years

and the doubts about the necessity of such weapons made me wonder whether Australia was called in to give financial support to a project whose cost were spiralling out of control. So. Do the financial reasons mattered a lot more than the military reasons to get in the project?

Note: A further detail about the project. I do not know whether this is the open issue. But it is enough to raise some doubts. The submarines that Australia is expected to acquire should be powered by the Rolls-Royce PWR3 reactors. Those reactors are an evolution of an American S9G reactor. According to Wikipedia:

the MoD awarded Rolls-Royce a £600 million contract to produce reactors for the Dreadnought-class and also for the final boat of the Astute-class

It means £120 million per submarine (4+1) Not only it is an expensive project, it is behind schedule:

the National Audit Office reported that the construction of the plant was five years behind schedule

Answer 1

Almost Certainly Not

What leverage does the UK have over Australia that could cause them to act against their own self interest? - especially in terms of military readiness, where states tend to be at their most "selfish."

Capabilities

Diesel Submarines and Nuclear Submarines are very different in terms of their capabilities. Diesel submarines are very quiet when they are on the battery, but they are slow to re-position and loud when dieseling to recharge or travel on the surface. Basically, diesel boats are very useful for coastal defense, but cannot project power abroad at all.

A Nuclear sub has the endurance, stealth, and speed to threaten ships or shore facilities far from home. This provides a viable counter to spreading Chinese influence in the Asia-Pacific that a diesel boat cannot.

Win-Win-Win

The most likely answer is a win-win-win deal:

• The US saw a chance to gain a deterrent against China at the low cost of sharing nuclear tech with a close ally
• The UK saw a chance to lower the per-unit-cost of their new ship by sharing the design with the a close ally
• The Australians saw the opportunity to field a far more capable vessel without investing in a massively expensive indigenous nuclear program

It is possible that Australian thoughts on what "threat level" China presented changed between 2007, when the Collins class replacement process started, and 2021 when AUKUS was signed. The perception that China was more aggressive in the region would make a more credible threat more attractive to the Australians.

Behind Schedule?

OP references a reactor as being behind schedule, but it's important to read the rest of the sentence in that Wiki entry:

In January 2020, the National Audit Office reported that the construction of the plant was five years behind schedule and was now forecast to be in service in 2026.

So the Audit Report was nearly two years old when the AUKUS deal was signed. Working reactors in 2026 seems consistent with the planned late 2020s (UK) and early 2030s (Australia) deployment dates for the ships.