This failed to impress the activists, who erupted with demands: ''Well, resign. Get out!''

Almost four decades later, a frail Dexter reviews footage of the show. ''There's a row of all the wild-heads, sat right behind me,'' he observes.

''Yeah, including me,'' says Gary Foley, a noted Aboriginal activist.

Filmmaker Haydn Keenan has brought Dexter and Foley together for this remarkably congenial meeting. Keenan has amassed reams of formerly secret files, photographs and surveillance footage from the Australian Security Intelligence Organisation dating back to the 1950s, '60s and '70s. At the time of the Frost telecast, ASIO was worried that communists were cultivating Aboriginal agitators and stoking their rage to provoke violence against the state. With access now to his security file, Foley realises that ASIO believed he was capable of such violence.

So he tells Dexter: ''I think it was extraordinarily courageous of you to walk into a lion's den.''

More courageous than Foley could have known. Dexter reveals: ''Frost had actually invited the minister [Jim Cavanagh], who had been very disturbed by a report that he was going to be assassinated at the Frost show. He asked me to go instead, so I did.''

But Dexter had been identified as a potential target, too. He and his family were placed under 24-hour police guard for two years, and he discloses that he felt it necessary to play a ''double role'' and feed information to ASIO. He reads from a confidential letter in which he reported that an Aboriginal staff member had ''heard talk'' of a plot, allegedly involving Foley and fellow activist Paul Coe, to blow up the MLC Tower that housed his department in Woden, Canberra.

As a young firebrand, Foley had declared publicly a willingness to resort to violent struggle, but he tells Dexter that the Black Caucus never considered armed revolution a realistic option, and that the Communist Party of Australia had no more hope than ASIO of infiltrating the movement. The tower was never bombed. There was no assassination.

''In a sense it sort of hurts me to think you thought we would want to kill you. To this day, nobody can pin anything on me of having ever done, you know, anything violent.''

It all remains, nonetheless, in his ASIO file.

This riveting scene features in the third episode of Keenan's four-part documentary series Persons of Interest, which begins on SBS on Tuesday night. The other episodes focus on the journalist Roger Milliss, whose father, Bruce, was a secret member of the Communist Party while he was election manager for Labor prime minister Ben Chifley; the 1960s Monash University activist Michael Hyde; and the author and outspoken communist Frank Hardy.

The ASIO that watched these men was a fraction of the organisation it is today, but it made time to open files on many thousands of ''persons of interest'', including a parade of Australians who held or rose to esteemed positions: the future premier of Tasmania Jim Bacon, Reverend Ted Noffs, poet Kenneth Slessor, painter Lloyd Rees, writers Christina Stead and Christopher Koch, actors Peter Finch and Leonard Teale, Dr Fred Hollows, journalists Alan Ramsey and Paddy McGuinness, feminist author Germaine Greer, Aboriginal leader Eddie Mabo and a debonair young David Stratton, now a TV film critic of tireless charm but then the director of the Sydney Film Festival who screened movies from the Soviet Union.

Retired High Court judge Michael Kirby discovered ''with horror and shock that my Aunty Glory - according to my ASIO file - she said that I was brainy but a reactionary''. Former Labor politician Meredith Burgmann found her younger self described as having blue eyes and a good suntan but she looked ''cheap and untidy, smokes heavily''. Keenan mines the security agency's archive of surveillance film and photos that placed targets at demonstrations - anti-Vietnam War, anti-apartheid, land rights - ''causes that ASIO regarded as communist threats and which we now regard as just''.

But how good was ASIO's information? Not very, according to Foley. From his file he learnt of ASIO's suspicions that he was meeting a communist contact for a trip to Gympie. He noted the woman's name. ''And I thought, 'I remember her,' and then I realised this wasn't about politics at all, folks. This was about sex. We were sneaking off … for a dirty weekend.''

Former ASIO agent Tom Shepherd, who had volunteered as a university student to help his country, but who now regrets his role, tells Keenan: ''Agents would gild the lily just to make themselves interesting and feel that they're fulfilling what their handlers required because, again, of this constant pressure of, 'There must be something else.' It doesn't matter what it is. 'It may be true, it may be untrue, but it's better that we know about everything that you have heard, seen, and then we'll decide what to use and what not to use.' ''

Frank Hardy's 1950 novel Power Without Glory attracted a charge of criminal libel brought by the wife of underworld figure John Wren, but when a jury acquitted the author, the director-general of ASIO, Charles Spry, requested a list of the jurors and, should they have files, a summary ''would also be appreciated''. Robert Menzies, as prime minister, requested security checks on applicants for literary grants.

A 1966 photograph shows Hardy staring into the lens at Sydney Airport, but he was oblivious to the photographer. Hardy was greeting the Soviet poet Yevgeny Yevtushenko, and he would show him a good time in Sydney. An ASIO agent followed them to the Chevron in Kings Cross where Rolf Harris invited the poet to join him on stage with Jan Rennison, a former Miss Australia and recent James Bond girl. Yevtushenko refused, declaring it beneath his dignity. But the ASIO agent recorded that Hardy needed to physically restrain the poet from manhandling the beauty queen. She had spurned his advances and Yevtushenko had yelled: ''You will not make love with the greatest poet in the world.'' In the case officer's assessment, the Russian was a ''drunken, lecherous, conceited guttersnipe''.

''Typical night out with Frank Hardy,'' says his granddaughter, the author Marieke Hardy, reading aloud from the ASIO file. She notes further gratuitous editorialising in an ASIO officer's assessment that a 20-part television comedy, which the ABC commissioned Frank Hardy to write, would be a ''flop''. The officer wrote a ''delicate source'' had pointed out that Hardy was a close friend of Clement Semmler, the ABC's assistant general manager, and said: ''Quite apart from the fact that if Mr Hardy has any sense of humour he is the only commo who has … ''

ASIO did have sound security reasons to follow some targets. Michael Hyde confirms the ASIO intelligence on plans by leaders of the Monash University Labor Club to stage a violent July 4 demonstration in 1968. ''We were going to make it as militant as hell,'' he tells Persons of Interest. ''We organised smoke bombs. We organised rocks to smash up the consulate. I don't know how I feel about saying this now. I mean, I don't know whether the statute of limitations, how long it is. But … at a meeting of these leaders the day before the demonstration, a proposal to use petrol bombs to burn the US embassy down was defeated by only one vote. ''

Since the September 11 terrorist attacks of 2001, ASIO's budget is five times higher in dollar terms and its staff numbers have tripled to almost 1800. ''There's more surveillance happening now than ever before,'' says Keenan - and more than we know even from the CIA whistleblower Edward Snowden, he reasons. There is no better time, he believes, to consider the lessons of ASIO's mistakes and its overzealous surveillance of Australian citizens.

Australians who suspect they have a file can now request to see it 20 years after any intelligence was gathered. Until recently it was 30 years.

''It took me about three years to uncover the ASIO surveillance movies,'' Keenan says. ''There seemed to be a strange silence regarding the existence of what turned out to more than a hundred hours of extraordinary film. When I did find it, it was apparent that while the films were now of no security value they were the most incredible archive - a treasure chest of Australian social and political history.''

Not only did they document the history of dissent in Australia but also the social changes. ''Braces go out, skirts go up, long hair comes in, feminism arrives. The films are a unique insight into the changing face of Australia.''

To his horror, Keenan discovered ''someone inside the National Archive of Australia recommended to ASIO that they could sell the films for the value of the silver content they contained''. At least some of its surveillance film was sold this way.

''I regard this as an act of criminal cultural negligence. The films were recorded on a domestic video camera and then sent to Sydney to be destroyed. So, in a number of cases, all we now have is a shadowy amateur representation of the original. I suspect that's why someone wasn't keen for me to find all the missing films. I say heads should roll for this criminal destruction.''

Foley says ASIO ''has done us a bit of a service'', inasmuch as he can use his file to piece together so much lost from memory. While he insists he did not plan to hurt anyone, there was an altercation with a superior that led to his dismissal from the Department of Aboriginal Affairs. Foley was the only person Barrie Dexter ever sacked.

''You thought I was a proper bastard, didn't you?'' Dexter suggests at their meeting.

After all these years, Foley is gracious. ''If I was you, I would have sacked me too.''

But Foley also believes that if he dared to utter today some of the provocative remarks he made at 19, ''I'd probably be in an orange jumpsuit and rendered to Guantanamo Bay. But that's for the next generation of people with a conscience to worry about.''

■ Persons of Interest starts on Tuesday at 8.30pm on SBS. Visit http://www.smartstreetfilms.com.au/ to watch more ASIO recorded footage.