The Australians will now have to complete another set of practical learning, which will include spending time on retired nuclear-powered subs known as moored training ships.

After that, they'll receive further training in Connecticut before being assigned to a Virginia-class sub.

"The plan at this stage is to join submarines based in Pearl Harbor, Hawaii, and they'll complete their training at sea," Vice Admiral Hammond said.

"Ultimately, we need them to pay their skills forward.

"So once they're qualified, sufficiently experienced, then we'll get them back into the ecosystem in a different role."

AUKUS presents major workforce challenges for Australia

The AUKUS plan, announced in San Diego earlier this year, will see Australia acquire a total of eight nuclear-powered submarines at a cost of up to $368 billion. 

US submarines are increasing their visits to Australian ports from this year, and from 2027 HMAS Stirling naval base in Western Australia will host rotations of American and British subs under what's known as 'Submarine Rotational Force-West'.

Australia is expected to buy at least three Virginia-class submarines from the US from the early 2030s, before building its own nuclear-powered boats in Adelaide to be known as SSN-AUKUS.

They will be based on a British design using US technology, with the first scheduled to be delivered in the early 2040s.

Retired submariner and senior fellow at Washington-based think tank the Hudson Institute, Bryan Clark, described it as a "crawl, walk, run" approach.

"The biggest challenge is going to be transitioning from having some Australian-owned, US-built submarines … to having an Australian-built or at least a purpose-built Australian nuclear submarine," he said.

"It's going to require a massive amount of infrastructure, incredible workforce demand, both in terms of technical skills and numbers.

"It just seems like that's going to be a pretty heavy lift on the part of Australia to do nuclear ship construction."

The federal government says AUKUS will create 20,000 jobs over 30 years across the Australian Defence Force, the public service and industry, including roles such as tradespeople, engineers and project managers.

The number of Australian submariners will also need to be increased, with nuclear-powered submarines carrying larger crews and requiring personnel trained to operate the reactor onboard.

"The submariners that come out of Australia are very smart, very capable, fully able to take on that challenge of becoming nuclear plant operators," said Mr Clark, who is also a former executive officer of one of the moored training ships in Charleston.

"The difficulty might be in getting the numbers that you need to be able to man a nuclear submarine."

Virginia-class submarines carry around 132 people, nearly three times the size of the crew onboard the Collins-class boats Australia has now.

And unlike the Collins, nuclear-powered subs do not need to surface regularly to recharge, meaning they can stay submerged for months at a time.

Vice Admiral Hammond acknowledged the scale of the task confronting the Navy.

"We were already focused on recruiting, increasing the size of the submarine force and then initially bleeding across in smaller numbers into the nuclear power program and then scaling up as we go," he said.

"So it's a complex challenge, especially given the workforce environment back in Australia.

"Every company, every organisation wants talent. So we'll be focused very, very clearly on recruiting and retaining."

Could Australia set up its own nuke school?

More Australians are expected to follow in the footsteps of the first three graduates in Charleston, while Navy personnel are also training in the UK.

But Vice Admiral Hammond said Australia could eventually host its own training program.

"I think if we're serious about developing a sovereign nuclear submarine capability, then in time, definitely, all parts of the ecosystem built and operated by Australians in Australia, that should be the aim point," he said.

"But we don't need it all at once."

Asked where it could be located, he said the "sensible approach" would be either Adelaide or Perth.

The new subs will be built in South Australia, while Western Australia's HMAS Stirling is undergoing an $8 billion expansion.

[Fleet Base West, just south of Perth, known as] HMAS Stirling is Australia's largest naval base. (Supplied: Royal Australian Navy)

"They will be the two centres of excellence, if you like, for naval nuclear propulsion in Australia," Vice Admiral Hammond said.

"If you cast our minds forward, probably another 10, 15, 20 years, then the majority of the nuclear-trained submariners will be in the home port of the submarine force.

"There'll be a large number in Adelaide, but most of the boats won't be in Adelaide. So through that lens, probably WA.

"But that's a decision for governments and probably a decision for next decade, I'd imagine."

Challenges lie ahead to bring AUKUS to fruition

Aside from skills and workforce issues, there are other major challenges that still need to be overcome to bring AUKUS to fruition.

The sale of Virginia-class submarines to Australia requires the approval of the US Congress, and significant changes are needed to a complex set of export controls restricting how sensitive technology is transferred.

Questions also remain over how the US will deliver the promised Virginia-class submarines, given the pressure its own shipyards are under to meet local demand.

The US Chief of Naval Operations, Admiral Michael Gilday, last month said it was "too early" to provide an answer on exactly where the subs would come from.

"I wouldn't expect them to start identifying submarines by name or by hull number just yet; we've got time to work through that," Vice Admiral Hammond said.

"But at the moment, there is a deep tripartite commitment to doing this."

Lieutenant Commander Heydon, Lieutenant Commander Klyne and Lieutenant Hall will now move onto practical training.(Courtesy FARRAH TOMAZIN via Sydney Morning Herald)