But he would never have accepted the job if he had known the toll it would take on him and his family.
"Indeed, had I glimpsed just a little of the suffering I would witness and the heartbreak I would endure, I would have fled in the other direction," he writes in his book, God in a Brothel: An Undercover Journey in Sex Trafficking and Rescue.
About two million children are forced into prostitution every year.
Walker says many of those children are either kidnapped or coerced into brothels and told they cannot leave unless they pay a huge cash "debt".
His job meant heading into brothels equipped with a tiny camera and a false identity as a sex tourist or sex tour operator.
He would hire a woman or girl he suspected of having been trafficked and take her to a private room.
Once inside, he would make up an excuse for why he didn't want to have sex and ask her how she came to be in the brothel.
The testimonies he gathered from the women and children and the footage he took inside the brothels were collected as evidence and handed to local authorities.
In one case, 99 women were freed from a brothel in Southeast Asia in a raid carried out because of evidence he had gathered.
Many of the girls taken out of the brothels he infiltrated were as young as five or six.
Despite those successes, Walker started feeling overwhelming pressure to do more.
He became "haunted" by the faces of the people he had not been able to get out of the brothels.
That included one "terrified" woman who gave him vital evidence in the lead to the police raid that rescued 99 women.
When the raid happened, she wasn't there.
"I still remember her because she took her ring off and said to me: 'You will remember me by this'," Walker says. "I still think of her."
His devotion to his work and long stints away from home were making him increasingly estranged from his family.
That reached crisis point when he met a woman called Katya in Eastern Europe at the end of a six month deployment.
Believing she had been coerced into prostitution, he took Katya, who was in her late 20s, back to his hotel to talk.
After finding out she had not been forced into her job, he had sex with her.
"I was a committed husband and believed in the sanctity of marriage, in faithfulness and fidelity. This was who I was.
"Within the space of five seconds, I betrayed all that I was, all that I stood for and believed in as well as my marriage vows, my wife my integrity, my role, my very life," he writes.
Walker says he was "completely and utterly broken" by his infidelity. It cost him his marriage and his job.
His mistake led to months of counselling and soul searching, where he had to confront his failings and eventually forgive himself.
Now he has started his own organisation called Nvader, which is also aimed at combating sex trafficking.
Its workers never do deployments of more than two weeks and receive counselling after every trip away.
Walker says the failings he found in his own work have helped him to create a better system for others.
The facts about child sex tourism:
Most child prostitutes are under the age of 12.
A child prostitute can serve up to 1500 clients a year.
Every year, child sex tourism victimises about two million children
Up to 90 per cent of children rescued from brothels are infected with HIV
Eight out of 12 child prostitutes will have an abortion.
Daniel Walker will be carrying out a live chat on nzherald.co.nz at 12pm today.
* Daniel Walker is a pseudonym adopted for legal and professional reasons.
