A former ASIO spy is breaking a 46-year silence to reveal what she says is the darkest secret of all, something which ASIO and successive governments have never admitted in public.

"I have no doubt at all that ASIO was penetrated," Molly Sasson told 7.30.

"The Soviets always seemed to be a step ahead of us. If we put on an operation, it failed.

"There must have been a tip-off. It can't have been otherwise."

After working for British intelligence during World War II, and then MI5 in the post-war period, Ms Sasson was offered a job with ASIO in Canberra by its then chief, Charles Spry.

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Ms Sasson was given one of the most sensitive jobs in ASIO, to compile the daily intelligence report on Soviet diplomats who the agency suspected were operatives for the KGB or GRU, Soviet military intelligence.

"I used to get all the reports from the intercepts, from agents' reports," she said.

"I saw the static sight reports and had the surveillance team behind me and from that I got a pretty good idea of what they were doing."

Molly Sasson grew suspicious when a Russian agent she was tracking quickly fled the country. ( Supplied )

Former defence intelligence analyst Paul Monk backs Ms Sasson's assessment.

"She was in pole position to learn what the organisation was learning and compare that to the follow-through on operations, which on her account went often astray or were aborted," Mr Monk said.

At first Ms Sasson put the failure of ASIO's counter-espionage operations against the Soviets in Canberra down to incompetence.

But as operation after operation failed, she gradually came to a more sinister conclusion.

One operation in particular raised her suspicion.

She was chasing a suspected Russian agent who she believed was going to meet an Australian contact in a Canberra park.

"We were waiting for this particular man to come at 6:30 that night and they were all in place and I was in the office waiting," she said.

"But they never turned up, he never turned up. He took the evening flight to Moscow.

"Then we found out that he had travelled from the embassy in the morning and drove to Sydney and caught that flight.

"So somebody tipped him off and he was keen to get out."

Fears of Soviet mole inside ASIO dismissed

Ms Sasson was in no doubt that there was a Soviet mole working in ASIO.

She confided her fears to her immediate superior, ASIO deputy director Colin Brown, but those fears were dismissed.

"He said to me, 'Well, don't open this can of worms'," she said.

In 1974, Ms Sasson also revealed her suspicions to Justice Hope during the royal commission into ASIO.

She was scheduled to have half an hour with Justice Hope, but ended up getting two hours.

Former defence intelligence analyst Paul Monk says the KGB identified Australia as a crucial target in the 1970s and 1980s. ( ABC 7.30 )

There she handed over six sheets of evidence to him, which contained the names of several ASIO colleagues whom she suspected of being too close to the Soviets.

The royal commissioner reached no firm conclusion, finding only that "ASIO may or may not have been penetrated by a hostile intelligence service".

Mr Monk said the KGB identified Australia as a crucial target in the 1970s and 1980s.

"It's important to realise we're a key part of an international security and intelligence network," he said.

"The key players have always been the United States and Britain, but we've been closely linked into it since the beginning of the Cold War.

"What the Soviets set out to do was to get access to British and American intelligence, rather than simply ours, by the back door.

"And it's been stated that they were so successful that they were getting access to everything via their assets in Canberra."

The Americans knew that ASIO had been compromised.

Ms Sasson recalls a dinner conversation in the mid-1970s with the then CIA station chief in Canberra.

"I said to him, 'Why don't you join us for some operations?' she said.

"He looked at me and smiled and said: 'We can't do that, you're penetrated, we know that'."

Neither ASIO nor the government have ever admitted that ASIO was compromised during the final decades of the Cold War.

But Paul Monk said he believed it was important that Ms Sasson was speaking out.

"For her to come out now and say this needs to be on the record, I think, is path-breaking and I would like to think this will set the ball rolling for the matter to be cleared up in the public domain, so it's no longer a matter of speculation or dark suspicion but of confirmed and seriously reviewed history," he said.

But Ms Sasson said she just wanted to set the record straight.

"It's very, very important for ASIO to be fully believed. If they can confront their past, and do it properly," she said.

And Mr Monk said it was also important for ASIO's future.

"If it happened during the Cold War, why would it not happen again?" he asked.

"If we haven't got adequate accounting for what went wrong before, what warrant do we have for believing now that the organisation set up to protect us and our security and our place in the international alliance will function now, and not be compromised?"