That Stuxnet happened was fairly well documented, he says. But as he dug around he found "it was one of those subjects nobody wanted to talk about".

Stuxnet was the name given by security experts to the worm that made its way into an Iranian plant in 2010 in a co-ordinated cyber attack on the country's nuclear program, causing equipment to malfunction and briefly raising the spectre of a reactor meltdown.

This attack – and its far-reaching implications – is the subject of Zero Days, the latest film from the prolific documentarian who has directed 17 features since winning an Oscar for Taxi to the Dark Side in 2008 (among them Wikileaks: We Steal Secrets, Going Clear: Scientology and the Prison of Belief, Freakonomics, The Armstrong Lie and Finding Fela).

The American and Israeli secret services are widely suspected to have been behind the attack, which is believed to have begun when a spy smuggled a USB stick into the nuclear plant at Natanz. But no one has ever officially taken responsibility.

That secrecy is, says Gibney, one of the biggest threats posed by the emerging practice of cyber warfare.

"It's very difficult to do attribution in this area – so the possibility of carrying out retribution against the wrong people is very high."

It's also very difficult to quarantine an infection that by its very nature wants to spread, as the Americans found.

That USB stick may have been the patient zero of Stuxnet, but pretty soon it was behaving like the Ebola virus, says Gibney.

"It didn't just spread into Natanz – it also spread outwards from nearby IT firms to the world beyond Iran. US Homeland Security was terrified because there was this very powerful virus infecting American computers, and they suspected some dastardly nation state was behind it – not dreaming, of course, that the dastardly nation state was right across the street."

Zero Days does a credible job of bringing this tale to life, despite some substantial hurdles: the geeks know the story already; the non-geeks have no idea it ever happened; the people responsible won't admit it happened; and the people who could talk about it will only do so obliquely (in large part, Gibney suspects, because the Obama administration has been so vigilant in prosecuting whistleblowers).

Oh, and there's also the fact "the main character is a computer virus", he adds. "Though it definitely has plenty of personality."

Gibney says Barack Obama's administration has been relentless in prosecuting leakers. Photo: AP

There's terrific material here for a conspiracy thriller replete with shady characters and shadowy acts, so it's perhaps inevitable that Gibney is developing it as a fictional TV series.

"We envisage starting with the Stuxnet story and then expanding into fictional territory," he says.

Has this leap into fiction been a long time coming?

"It's not something I had been seeking," he says. "I've been having a great time doing documentaries but in this case it seemed fertile territory. Because this world is so secret you can say things that you wouldn't be able to say in a documentary, and they will be truthful in some essential way even though they're not factually accurate."

In fact, he says, without the secrecy of the agencies behind Stuxnet there would be no leap into drama. "May they stay classified forever – for the good of the series, if not for democracy."

But cyber warfare is no joke. What Stuxnet demonstrated was that computer viruses could leap from the world of code to the world of objects, with potentially devastating consequences.

"People don't see it as a threat in the same way they would with biological weapons or chemical weapons or nuclear weapons. But when you're talking about crossing the barrier from the cyber realm to the physical realm, anything is possible.

"We had a train crash in New Jersey recently that was a bit strange because the train sped up just before it hit the station. You could say that's just what happened, but it could also have been a cyber attack – because once you cross over, there is a tremendous amount of destructive power that is possible."

Oscar-winning documentary maker Alex Gibney. Photo: Joe Armao

So, now this genie is out of the bottle, is there any way it can be put back in?

"No, it can't be put back in. The genie's out. It's a Pandora's box. It's all those metaphors and that's why the film talks about how important it is, just like with nuclear weapons, that people start thinking about rules to govern the use of these weapons.

"Unless people fess up [to doing it] and start figuring out rules for the road, bad things are going to start happening everywhere. And some of the most powerful nations, particularly the United States, are the most vulnerable because of the degree of interconnection they've already put in place."

Zero Days screens at ACMI, Federation Square from October 13 as part of the Lies and Secrets series of documentaries about clandestine government activities. Details: acmi.net.au/lies-and-secrets