A group of prominent businessmen, including Dick Smith and John Singleton, have taken out a full-page ad in The Australian newspaper, suggesting the public is being conned over the submarine project.

French company DCNS won the $50 billion contract to build Australia's next fleet of 12 submarines in Adelaide, which will replace the current Collins Class fleet.

The company won the contract to build a modified version of its nuclear submarine, called the Shortfin Barracuda.

The Australian Government stipulated that the winning contract would need to use conventional power, ruling out larger, nuclear-powered submarines.

Mr Smith said the re-designed version of the submarine would have to be converted to a diesel engine.

But he told 891 ABC Adelaide that was a ludicrous plan and he believed it would never happen.

"So the plan is for us to buy a nuclear submarine design and then convert it to a piston submarine," he said.

"Now no-one has ever done that in the world and in fact when I talk to submarine experts they say it is so ridiculous, so we're being conned."

Mr Smith said if the Government's real agenda was to use nuclear technology, it should be up front about it.

A Federal Government spokesman said the "best experts" were involved in the Government's decision-making on the project.

"These submarines will be regionally superior. They will allow Australia to pursue our national and international interests and fulfil our role as an effective US ally," he said.

"Building the submarines in Australia has immense benefits for our economy and in creating defence industry as a fundamental input to capability."

More details on subs project needed: Xenophon

South Australia Senator Nick Xenophon said more detail about the submarines project was needed.

But he questioned the motives behind Mr Smith's campaign.

The advert that was placed in The Australian newspaper. ( ABC News )

"I think the criticisms of the uncertainty are fair enough, but if the criticisms are about a campaign not to build the subs in Australia and to scuttle project then perhaps Dick Smith and I will be on the other side of the barricade," Senator Xenophon said.

"If you speak to submarine experts, a diesel sub with electric motors, in other words, with battery power is actually quieter than a nuclear sub so it's really a case of what you're actually looking at and what you're looking for.

"There needs to be more details of the program, we need to know when the program will be up and running in terms of contracts being signed."

Senator Xenophon said he was concerned about the lack of certainty surrounding the project.

"There are some quarters in defence spaces saying, 'look, this is going to be complicated, we are going to have a capability gap'.

"I would have thought by now we would have had much more certainty after the April announcement."

South Australia's Defence Industries Minister Martin Hamilton-Smith said he trusted the decision.

"I think the Navy chose that rather than the Government and you have to trust their judgment," Mr Hamilton-Smith said.

South Australia Premier Jay Weatherill hit out at Mr Smith on Twitter, calling the businessman a "sad old man".

"Looked like it [the advert] was scribbled on the back of a serviette after a long lunch - #sadoldmen," Mr Weatherill tweeted.