STEVE CANNANE, PRESENTER: They call themselves online activists, the growing legion of cyber detectives who take on cases that they feel society and authorities are neglecting.

Some see them as heroes, others as vigilantes.

The most notorious among them is the group calling themselves Anonymous, who after the attacks in Paris earlier this month declared they would use the web to strike ISIS and its communications network.

Alison Caldwell reports.

ALISON CALDWELL, REPORTER: In May last year, a neo-Nazi rally in Brisbane was interrupted.

A group of construction workers from a building site nearby had been tipped off about a demo supporting Greece's Golden Dawn party. Without warning, a counter-rally sprang into action.

The counter-rally wasn't the unionists' idea. It was organised by a man on a computer nearly 2,000 kilometres away.

'ANDY FLEMING', CYBER ACTIVIST: What I do is try and monitor the activities of the far-right in Australia and the events that they organise.

ALISON CALDWELL: 'Andy Fleming' is a cyber activist who trawls the web for information and cultivates sources inside the organisations he's targeting. That's how he found out about the planned neo-Nazi demonstration.

'ANDY FLEMING': I published a Facebook page asking for people in Brisbane to go along to oppose their rally. Fortunately, a number of unions had a presence of several hundred of their members. I was able to provide them and others with information about who Golden Dawn was, an anti-union neo-Nazi organisation. They agreed that it was in their interest to oppose them, so they did. I think what happened in the end is the Nazis had to escape in a taxi driven by an Indian taxi driver from the event.

ALISON CALDWELL: 'Andy Fleming' is one of thousands of online activists around the world who are using the internet to right the wrongs they see in society. He says he's received death threats and doesn't want to appear on camera.

It's not enough for him to write letters to the newspapers or lobby politicians for change.

'ANDY FLEMING': I found it useful to obtain information about these groups which can be used at opportune moments when and where it becomes necessary.

ALISON CALDWELL: Do you find that it's more effective, a much more effective way to achieve what you're after?

'ANDY FLEMING': I think so. I think I've been a disruptive influence. And I should also add that I'm working with a range of other people who belong to a kind of loose network of people who do the same kinds of work.

ALISON CALDWELL: The successes of WikiLeaks and Anonymous have spawned tens of thousands of other online activists, with web sleuths and citizen detectives now challenging governments and security agencies around the world.

McGill University Professor Gabriella Coleman is recognised as one of the world's leading authorities on hacker culture and online activism.

GABRIELLA COLEMAN, AUTHOR: I guess at some level, cyber sleuthing and cyber detectives can be considered as a form of activism, but it is a very particular form where citizens are deciding that there is something wrong or there's some information that if they gain, they can locate perpetrators or put an end to paedophiles.

ALISON CALDWELL: She believes what's happening on the web will have serious consequences for policing and justice.

GABRIELLA COLEMAN: I think it's undoubtedly that they will have to shift practices because of the fact that many different citizens all over the world are deciding to take action. For example, let's just say with all the activity in catching people paedophiles. If this is kind of successful, it kind of shines a light on law enforcement that they may not be doing their job.

ALISON CALDWELL: Earlier this month in Melbourne, a group calling itself The Tinder Experiment drew praise and scorn after it posted videos online of older men apparently preying on a woman posing as a 15-year-old girl.

Using hidden cameras, the group filmed men who'd responded to a match on the dating app Tinder suggesting that Imogen, aged 15, was nearby. In fact, Imogen is 21 years old. She doesn't want to appear on camera because she says it may compromise future experiments. But to look at her, she could easily pass for a teenager.

IMOGEN, TINDER EXPERIMENT PARTICIPANT: I wanted to see how this would pan out and, yeah, basically just do an experiment and I trusted the people I was working with anyway.

ALISON CALDWELL: Blake masterminded the so-called experiment.

BLAKE, TINDER EXPERIMENT PRODUCER: We felt that there wasn't enough awareness and that people didn't really know that this kind of stuff went on and we wanted to just share that with everyone because it's not really well known.

ALISON CALDWELL: In the video Blake, playing Imogen's brother, comes home and feigns outrage.

Even after they were caught, Imogen says some of the men approached her again wanting to meet up in secret.

IMOGEN: They still went back again after the first time being busted and knowing how serious that what they just did was.

ALISON CALDWELL: After the Charlie Hebdo shootings in Paris, Anonymous declared war on ISIS, saying it would strike at the heart of its communications network.

According to Professor Gabriella Coleman, this is no hoax.

GABRIELLA COLEMAN: This is one of the interesting operations that tended to be both controversial, but also draw a tremendous amount of support, both from an English-speaking population and especially a French-speaking population as well.

ALISON CALDWELL: Like a juggernaut, online activists and citizen detectives show no sign of backing off, and when police and governments respond with more surveillance, it only galvanises their efforts.

Those who have spent much of their waking hours listening to online chatter say police will have to change their approach to citizen detectives.

GABRIELLA COLEMAN: I think that's completely possible in the future, that they will open the door to selectively working with groups who do contribute to providing useful information for law enforcement.