He also has a podcast with comedian and actor Rhys Darby, best known as the band manager in Flight of the Conchords. Called The Cryptid Factor, it investigates the world of cryptozoology, the pseudoscience that tries to prove the existence of creatures such as the Loch Ness Monster, Bigfoot, Chupacabra and the Mongolian Death Worm.

So when a friend sent Farrier an email with a link to a video showing something called competitive endurance tickling, he thought he'd found good material for a two-minute news item. He sent an email to Jane O'Brien Media, the company behind the videos. And that's what opened a can of Mongolian Death Worms.

The response he got was negative and vitriolic. They warned him off investigating further, threatened legal action and were abusive about his sexuality – Farrier is a 33-year-old openly gay man.

"When I first saw the videos, I was unsure what I was watching," says Farrier, speaking from Darby's house in Los Angeles, where he is currently based. "You had these good-looking young guys in adidas gear being restrained and tickled by other good-looking young guys. But it wasn't a smutty video in a dank basement. It was all very professional, filmed in hi-res in a nicely lit studio. I didn't know if I was watching something sexual, or some kind of weird tickling league, or something else."

It turned out to be something else. Farrier quickly realised this was more than a short TV news item, so he started a Kickstarter campaign to make a film with co-director Dylan Reeve. They tweeted Stephen Fry about it. Fry not only re-tweeted it, but offered enough financial support to become the film's associate producer.

The result is Tickled, which was picked up by Magnolia Pictures and HBO in the US after it premiered at the Sundance Film Festival in January.

The more the duo dug into the tickling empire, the darker and creepier it became. Representatives from Jane O'Brien Media were sent to New Zealand to warn Farrier and Reeve not to investigate further. Things got ugly. The film-makers went to the US and gatecrashed a tickling filming session. Things got uglier.

They tried to track down participants in the tickling videos. Most of them were unwilling to talk, but those who did recounted tales of cash payments, free travel and gifts in exchange for filming, and then intimidation, bullying and smears to their reputation when they complained about the unauthorised use of the videos online.

It's difficult to talk too much about the documentary without giving the whole thing away, but it's pretty obvious from early on that Jane O'Brien, the woman who claims to be the person behind all this, is not who she says she is and that competitive endurance tickling has absolutely nothing to do with competition or endurance.

Tickled is so outlandish that at times you find yourself wondering if it's a very clever mockumentary.

"You're not alone in thinking that," says Farrier. "But I think if I wanted to make a mockumentary, I would have to be a lot smarter than I actually am. I just wouldn't have been capable of coming up with all this stuff. That's why I enjoy what I do and why I love watching documentaries myself, because real life is so much more bizarre than anything you could make up."

Farrier has a calm, inquisitive, slightly bewildered demeanour throughout the film, equal parts Louis Theroux and Bret from Flight of the Conchords. There are a couple of very tense scenes, especially when he finally comes face to face with the villain of the piece, but he still maintains his unflappable Kiwi reserve. Underneath it all, was he a bundle of nerves?

"I was ready to run," he says. "I have long legs that can carry me away quite quickly. In New Zealand, guns aren't an issue, but we were in America, and when you approach someone on the street, especially someone who mightn't want to talk to you, you don't know if they're carrying or not. That's an alarming thought. And I was thinking about nothing else at that moment."

Even after the film was done, the story continued. At a March screening at the True/False Film Fest in Columbia, Missouri, two private investigators from New York were sprung in the audience secretly recording the film on a device hidden inside a coffee cup. Farrier has been sued twice (both cases have been dismissed) and a tickling participant who agreed to talk on camera has been sued for $40 million. And in June, the head of the tickling empire attended an LA screening and angrily confronted Reeve.

Does Farrier ever wish he'd made a documentary about a less touchy subject, like, say, Scientology?

"The way they've reacted, it feels at times like I actually have made a Scientology doco rather than a film about tickling," he says, laughing. "But I'm glad Dylan and I made this thing. I'm proud of what we've done. I do wish the stressful aspects would decrease at some point, but I have to accept that. We were told pretty clearly when we started this project that we would be sued, so it's not like it was unexpected."

One thing that you won't see in the film is Farrier being put in restraints and tickled. Did it happen?

"It did, actually, with Richard Ivey, the tickling fetishist in Florida. Part of his deal with being in the film was that he would get to tickle me for 10 minutes. I thought he was joking, but he was serious. He's a professional tickler. He does it for a living. There was no safe word. It was pretty awful. I think even people who think they aren't ticklish, if they're restrained they find out they are. It makes it infinitely worse. At one point he had my shoes and socks off and he lubed my feet up with KY jelly and got one of those really spindly hairbrushes and was running it up and down my feet and it was the worst."

As for what's next, when he's not dealing with the legal fallout of Tickled, Farrier is in Los Angeles researching, writing and pitching ideas. Is it safe to say his next project will be something strange?

"Some of the ideas I've got are strange and some are not strange at all," he says. "But who knows? At this stage I've kind of lost perspective on what's strange and what's normal."

Tickled is in cinemas August 18.