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.