The young Swiss tourist was keen to check out the bars around his hotel, and the older Australian men looked like they’d be good company. They were politely declining offers of a massage from a gaggle of friendly young women outside “Merlyn’s Videoke Bar”, where a heart-shaped apostrophe hinted at more than just boozy singalongs. But when he suggested they go in for a drink, he’s floored by Tony Kirwan’s reply.

“We’re looking for girls a bit younger than this. Getting fussy in me old age.”

The backpacker’s eye’s widen and his smile fades; he vanishes quickly without saying another word. I imagine him back at his hotel, posting something on Facebook about his disturbing encounter with some Australian paedophiles. Seeing myself through his eyes, a little part of me died.

Nothing can really prepare you for the intense discomfort you feel when someone thinks you want to have sex with a child. But Tony Kirwan’s used to it — pretending to be a paedophile is part of his job description.

Sixteen years ago, the Queenslander founded Destiny Rescue, a Christian charity based in Thailand that rescues kids who’ve been trafficked into prostitution. In order to find them, he needs to enter a world most of us would prefer not to know about — a world in which kids are for sale.

Tony has invited me to watch him and his team at work, which is how I come to be in Manila tonight, freaking out a friendly backpacker. We’re trawling bars for underage sex workers in a red light district near the airport, wearing hidden cameras to ensure we can collect evidence to take to the police.

“At any given time, pretty much most nights of the week”, Tony told me, “we’d have a couple of dozen agents in six different countries, kitting themselves up, ready to go out into the brothels to get this sort of stuff on film.”

In the bars we visit, the music is loud, the drinks expensive, and the women bored. But although they seem young to a western eye, the age difference between them and the men they’re drinking with is more depressing than disturbing — Tony doesn’t think they’re under 18. And although his religious beliefs make him extremely uncomfortable with prostitution as a career choice, Tony’s focus is solely on identifying minors.

“Often it’s not the first thing you’re going to see,” explains Damien, his International Rescue Director, who’s also working the bars with us tonight. Prostitution is illegal in the Philippines , but largely tolerated — sex workers under the age of 18, however, are automatically considered trafficking victims, and their pimps can be jailed for life. Damien says that in order to find children, you sometimes need to access a shadowy underground, by making your forbidden desires known to the pimps, prostitutes and other patrons.

On the shabby stairwell of one particularly abject bar, the mamasan corners Tony to ask him why we’re leaving.

“We’re looking for …” he begins, hesitantly.

“What?” the mamasan quickly interjects.

“ … younger girls. I like really young girls.”

The mamasan thinks for a moment, and glances furtively around the empty stairwell, before asking, “How old?”

“Ah, 16 … 17,” replies Tony.

“We have, Sir,” she explains. “But we cannot talk … here. Maybe you have a cell phone? ”

Tony writes down her number and asks how many girls she can supply.

“Maybe 6 or 7 …. They’re not working here. It’s not legal for young girls to work here …. I have a friend who is their mamasan.”

The whole exchange is caught on hidden camera. Tony’s staff will share their intelligence with the under-resourced local police, and hopefully lure the pimp into providing the girls so they can be rescued. The Philippines National Police are grateful for any help — there are only a handful of officers in their anti-trafficking unit, which relies on support from NGOs like Destiny Rescue and World Vision to do its work.

At first glance, Tony might not seem cut out for this kind of work. Gentle and softly spoken, he’s not an ex-cop or a social worker — in a previous life, he was an electrician. (Among his staff are a plumber and a professional drummer — Destiny Rescue is a motley crew of mostly volunteers, driven by their social conscience and religious faith).

“I’m just a sparky,” Tony told me, with genuine modestly. “I’ve got no specific skills in this other than, I guess, the heart of a dad.”

In fact he’s had hurdles to overcome to do this work, including a sheltered upbringing. His parents didn’t even drink alcohol, and “to go from a very protected, wrapped-in-cotton-wool sort of life into this ugly side of things was a shock. I felt awkward even walking into a brothel.”

Then there’s his inhibiting shyness. “In the early days I found it challenging because I’m not a natural speaker, I’m a bit introverted, so I had to get over myself and push myself to talk.”

But Tony is selling himself short. It requires more than just a father’s protective instincts to rescue nearly 2000 children from sexual slavery over the past seven years alone. And rescue is just the first step — in order to prevent kids from returning to prostitution, there’s equal emphasis on rehabilitation, especially on the education and vocational training that will hopefully lead to a safer and more rewarding livelihood.

It’s been a steep learning curve, and Tony frankly admits to plenty of stumbles along the way, especially when it comes to so-called “soft rescues”, which are done without the aid of police.

“When we first started, as soon as we saw a kid in a bar we’d call them over and tell them who we were and we had 100 per cent of the girls say no. It took us about six months to work out what that problem was and it was they just didn’t trust us. I was blown away.”

He’d imagined a scene out of a Hollywood movie, climbing out a back window with a child in his arms. Instead, the recipe for a rescue turned out to be much more low-key — “build trust, make an offer, give them time to think about it, then pick them up and make it safe.”

And how do you build up trust when, on the face of it, you’re no different to the guys that abuse them?

“What we found is we had to go back multiple times, showing them that we are different to every other guy that makes them sit next to them. We don’t touch them bad, we don’t let them touch us bad. We basically treat them like they were our own daughters and by building that trust, then they can see that we’re good guys in a very bad place.”

I don’t think I could do this work. If you can’t simultaneously convince a child that you’re her protector, and convince her pimp that you’re a predator, then you’ve failed — and the consequences of failure in this line of work are awful to contemplate.

“You’re definitely role playing. To the bouncers or the mamasan, you want to look like a normal customer. To the child you want to look totally different to every other customer that she’s ever sat with. So it’s this balancing act of, of just acting all the time.”

Luckily for the children tracked down by Tony and his agents in places that no child should ever be, they’re good men and excellent actors.

Amos Roberts is a journalist for Dateline. Follow him on Twitter: @amosroberts

Watch the full report — How to catch a Pimp on Dateline, Tuesday 12 June at 9.30pm on SBS.