A private investigator, hired by Australian councils to investigate massage parlours offering illegal sex services, has revealed that he engages in sex acts to obtain proof that the law is being breached.

Speaking to the Daily Telegraph, the investigator from Lyonswood Investigations and Forensic Group said he had taken part in 50 investigations into massage parlours and was offered an "extra" 95 per cent of the time.

The man said: "'It's a job, I have absolutely no emotional connection to it. I go in, I have an objective to meet, I'm being paid to do it."

He revealed that on most occasions a sex act was performed on him but that he also engaged in full sex when the investigation required it.

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He told the Telegraph that the key to the job was to enter the premises confidently and give off the air of a regular customer.

He said that despite signs outside massage parlours saying they don't offer sexual services, many operated as de facto brothels and would offer sex immediately.

The man did not apologise for his line of work, saying: "'It's the only way we're able to show or demonstrate what's going on in there."

He said he needed to be absolutely certain that sex was being provided and told how he usually climaxed during the encounters.

The legal status of prostitution varies across Australia, with some of the most liberal laws being applied in New South Wales, where the investigator plies his trade.

It is legal to run a licensed brothel in New South Wales, though a 2009 report in the Daily Telegraph showed, at the time, illegal brothels outnumbered licensed outlets four to one.

In New Zealand, prostitution has been legal and regulated since The Prostitution Reform Act passed in 2003 but it remains a crime to coerce someone to provide sexual services and sex work is also prohibited for those on temporary visas.