It is close to the 40th anniversary of the unsolved disappearance of Australia's 17th prime minister, but the conspiracy theories will not go away.

The sensational disappearance of former prime minister Harold Holt, off a remote Victorian beach in December 1967, captured the world's attention.

But the lack of a body led to an avalanche of wild and persistent theories about Mr Holt's exit.

Many ascribed to Cold War scenarios - others had more earthy tones that he had faked his death.

Some theorists were so earnest they put pen to paper and alerted authorities, with the best letters now online after being released by the National Archives.

On December 17 of that year, Mr Holt went missing in rough seas whilst swimming at a favoured spot, Victoria's Cheviot Beach.

Police soon concluded it was an accidental drowning of an experienced swimmer in dreadful conditions, a theory that was confirmed in a belated Victorian coronial inquest in 2005.

But something did not quite add up for the conspiracy theorists.

A letter from an American lawyer, dated the day after Mr Holt's disappearance, reads: "My hunch from fragmentary press reports is there's a better-than 50 per cent chance that Mr Holt's death was not accidental, but resulted from expert sabotage, probably foreign."

It is a sample of the Cold War theories which can now be found on the National Archives website.

The author offers a couple of possible subtle methods that might have been used on Mr Holt, including: "Some delayed-effect drug, which he might have got in refreshments on his way to the beach."

"This would be revealed by expert autopsy, unless it's one of the new disappearing ones," the letter reads.

Other letter writers believed Mr Holt was still alive, and that he had either faked his death and run off with an alleged mistress, or was taken by the communists.

"I think that Mr Holt was kidnapped and taken away by submarines, and is being brainwashed for political information," one letter says.

"Our enemies know that Mr Holt and President [Lyndon B] Johnson were close friends. A word to the wise is enough - and I shall leave the rest to you and our government."

The National Archives has scores of Holt letters, with many collected by the Prime Minister's Department, or security agencies such as ASIO.

About one-third came from overseas, like the one from an astrologer who claimed to know where the body could be found.

Missing body mystery

Acting curator Tracey Clarke says the Holt mystery still intrigues 40 years on.

"I suppose the most obvious reason would be because there was no body that was found, so of course if there's no evidence of an actual death, that's going to create a lot of speculation about whether a person actually did die, or how they did die," she said.

She says the conspiracy theories may have emerged because Mr Holt disappeared during the Cold War.

"I think that's why one of the most popular theories was about Harold Holt being kidnapped for a political interrogation, and that one was one that has been circulating around for many years," she said.

Those who were closest to Mr Holt, including former press secretary Tony Eggleton, understand the interest in his disappearance, but stress the case has been solved.

"The most outrageous theory was the Chinese submarine," he said.

"It's long since been demonstrated that there was no way a submarine could have operated in those waters off Cheviot Beach.

"Anyway as [his wife] Zara Holt said, Harold Holt didn't even like Chinese cooking."

Mr Eggleton says that Mr Holt's demise was nothing more than a tragic accident.

"Basically I think that people just find it very hard to accept that a prime minister can go for a swim on a Sunday afternoon, like anyone else, and end up misjudging the situation and drowning," he said.

"But that's what happened on that afternoon 40 years ago."