When the going gets tough, they call on a senior citizen. :)

Three years ago Fred Allen* was a taxi driver working 12-hour shifts to make ends meet.

Today, he is a gun for hire, having received tens of thousands of dollars from Sydney's metropolitan councils in exchange for crucial evidence that is presented in court to help expose and close underground parlours. In short, Mr Allen has paid sex with prostitutes and ratepayers foot the bill.

"Never in a million years would I have imagined a job like this existed, let alone me doing it," the 60-year-old said, with a hint of a smirk. "It's a strange world for sure."

Mr Allen completed more than 60 jobs at locations across Sydney.

When Sydney-based Lyonswood Investigations advertised for a "brothel buster investigator" in 2011, it was inundated with resumes from as far afield as Finland.

But while all applicants were willing to engage in paid, undercover sex, the agency's managing director Lachlan Jarvis confirmed Fred was the only suitable candidate for the niche role. "He had his private investigator's license, his oral and written English was excellent, he was willing to appear in court if needed ... and he was single."

Mr Allen's maiden mission involved an undercover visit to an unlicensed brothel reportedly masquerading as a massage clinic. "I had never been to a brothel in my life so I was feeling quite nervous and apprehensive," he recalled.

"I didn't know what to expect. I reminded myself that this was a legal job exposing illegal activities. As far as first days at work go, I enjoyed myself."

Since then, a core group of approximately 10 Sydney councils have called on his services. "The drill is always the same, he explained. "An email arrives in my inbox providing the name, address and description of the premises. I then head in, get the information required and file a written report to the office, which is forwarded to the council."

Mr Allen confirmed he had completed more than 60 jobs at various locations across Sydney. In nearly every case, the establishments were "clean and comfortable" environments staffed almost exclusively by Asian girls who were in Australia to "study English". Sexual services were given in all but three of the businesses he has visited, he said.

"The jobs flow in, on average, once every three weeks. If it spreads out that way, it's perfect," he said.

"But there are occasions when they all arrive at once. For instance, I was given three jobs to complete, for the same council, in the same week ... and I'm not as young as I used to be."

While Mr Allen said he enjoys the thrill of going undercover, he doesn't believe there's a book in his adventures.

"I'd like to recount a series of hair-raising adventures and humorous anecdotes but, the truth is, it's all pretty run of the mill," he said. "I'm hired as your regular, everyday customer who walks in, requests a service, pays the money, and then leaves with a smile. I've never had a knife drawn on me or anything.

"I can assure you, it's far safer than being a taxi driver. It's better paid too."

Though their paths have never crossed, he is aware of one other agent like him in Sydney. Far from feeling threatened, he is "heartened" by the likelihood of there being more. "It would be nice to meet them one day," he said.

To date, he has only shared his secret with one other person: "I told one of my mates ... he was a bit incredulous and a bit envious, too."

While Mr Allen acknowledges his work is not the sort of job you want everyone knowing about, he has grappled with the idea of coming clean with his two adults sons.

"I'm in a quandary," he said. "I've considered sitting them down and telling them. Alternatively, when I kick the bucket, they'll go through my paperwork and discover for themselves.

"Either way, I hope they have a good chuckle."

* not his real name