Described as "only the beginning", the first initiative of the trilateral AUKUS security and delivery pact with the US and United Kingdom has been revealed as delivering three, possibly used, Virginia class nuclear-powered submarines every three years, followed by five trilaterally developed SSN-AUKUS subs. One is expected to be delivered in Australia every three years from 2042.

Prime Minister Anthony Albanese stood by US President Joe Biden and Mr Sunak at the naval base in San Diego on Tuesday to announce the "biggest single investment in Australia's defence capability in our history".

"Strengthening Australia's national security and stability in our region, building a future made in Australia with record investments in skills, jobs and infrastructure, and delivering a superior defence capability into the future," Mr Albanese said.

"My government is determined to invest in our defence capability.

"We are also determined to promote security by investing in our relationships across our region.

"This is a genuine trilateral undertaking - all three nations stand ready to contribute and all three nations stand ready to benefit."

The new fleet is expected to be complete by 2054. The submarines will be built at Osborne, South Australia, with an extra $2 billion spent in the state over the next four years. They will also be operated by the UK.

"This is a powerful partnership. For the first time ever, it will mean three fleets of submarines working together across both the Atlantic and Pacific, keeping our oceans free, open, and prosperous for decades to come," British Prime Minister Rishi Sunak said on Tuesday.

The subs will be nuclear-powered, not nuclear-armed.

"Australia is a proud, non nuclear weapon state and is committed to stay that way. These boats will not have any nuclear weapons of any kind," Mr Biden stated.

Australia and the UK are expected to begin work on the SSN-AUKUS in domestic shipyards within this decade.

Mr Albanese has pledged the subs agreement will lead to 20,000 direct Australian jobs, including many trades and specialisations, including specially trained submariners.

"The scale, complexity and economic significance of this investment is akin to the creation of the Australian automotive industry in the post-World War II period," he said.

He also regards AUKUS as a "catalyst for innovation" which will lead to research breakthroughs.

More Australians are expected to be trained in nuclear engineering through universities and research institutes.

The SSN-AUKUS is primarily based on a new British design with "cutting edge" Australian, UK and US technologies.

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The sale of the Virginia class submarines requires Congressional approval. It is first time in 65 years that the US has shared its nuclear propulsion technology.

The project will start this year with Australian military and civilian personnel embedding with the US Navy, the Royal Navy and with US and UK submarine bases.

Forward rotations of US and UK SSNs at HMAS Stirling in Western Australia, to assist building skills and operational capability, will begin "as early as" 2027.

The cost of the new subs program has been revealed as $9 billion over the next four years, $50 to $58 billion over the next decade and 0.15 per cent of GDP over its lifetime. This puts the lifetime cost of the AUKUS subs initiative between $268 billion and $368 billion.

"The important point to make about the cost in respect of the forward estimates is that defence is covering it. So defence is cost neutral with this over the forward estimates," the Defence Minister Richard Marles said in Canberra.

But asked where are the cuts coming from, Mr Marles was not forthcoming.

"You will get all of that information before the Budget, which is measured in just a couple of months, so you can judge us at that point," he told reporters.

The current defence spending is running at 2.0 per cent of GDP and it is expected to rise in the near term due to AUKUS and the outcome of the Defence Strategic Review.

It comes as Chinese President Xi Jinping declared that China must turn its military into a "Great Wall of Steel" to effectively safeguard his country's sovereignty, security and developmental interests.

In his first speech since becoming leader for a historic third term, Mr Xi told a gathering of Communist Party officials that China must modernise its military, oppose Taiwan pro-independence forces and promote "reunification of the motherland".

Described by Mr Sunak as an "unprecedented endeavour", the submarine plans come nearly 18 months after the AUKUS security and technology pact was first revealed under the former Coalition government.

On the eve of the announcement, Opposition leader Peter Dutton offered bipartisan support to pass budget savings needed to fund the "massive" program.

"The short answer is, yes," he told ABC's 7.30 program.

"There will always be points of difference. If there are different ways in which we can provide support to the government, we are happy to do that."

National security expert Professor John Blaxland said the deal was a "momentous" occasion for Australia and on par with historic events, such as the Snowy Hydro Scheme and Australia's bilateral atomic weapons research project with Britain.

"This is even bigger because this is tri-national," Professor Blaxland said.

"It is a multi-decade, inter-jurisdictional ambitious project that is fraught with many potential pitfalls, all of which are only surmountable because of the resolve that Beijing's actions have triggered in the UK and in Australia, alongside the United States."

Australia Institute's security affairs director Allan Behm is less convinced the project, which he said was being driven by "wild, unachievable ambition", could be delivered.

While Mr Behm agreed nuclear-powered submarines would be a helpful addition to Australia's armaments, he questioned how achievable it was for the country.

"There are good reasons why a lot of other countries have not acquired nuclear-propelled submarines. It's a very difficult thing to do and we're only the seventh country to do it," he said.

"We've got to build the industrial base that we need to ... build these submarines.

"If you think about that, just for a moment, the industrial base of Britain, the United States, France, China, Russia, they're huge industrial bases, which do a whole lot of other things, so the ability to build submarines is just another thing that their heavy industries can do.

"The curious thing is that [Australia] will end up with a command economy in the heavy industrial sector - an economy, which is driven only by a sole client, that is the government."