Investigator Luke Athens, the director of the International Intelligence Agency, based at Parramatta, said he charges up to $5000 to prove a massage parlour is a front for a brothel.

A Parramatta Council spokesman said there are 22 approved brothels or sex-related premises in the local government area but it would be difficult to determine how many illegal brothels there are.

He said the council does not need to approve every massage clinic which is opened in the area, because often they are just classified as commercial enterprises and don't require approval unless structural changes are made to a building to accommodate them.

Parramatta Councillor Michael McDermott, who has consistently called for the council to be more vigilant in policing brothels, said cost shouldn't be the only consideration.

He said the council had a responsibility to protect the area's residents from illegal brothels.

Mr Athens said many councils hire his agency to determine whether massage parlours are really what they represent themselves to be.

"This has been a trend for a very long time and councils have used private investigators to look into it for years," Mr Athens said.

"The problem is that it can be difficult to prove that the sexual activity is going on.

"Investigators need to go back a few times and get video evidence for the council to have enough evidence to shut the establishments down."

At the last council meeting Cr McDermott read out an email from a young woman who had attended a massage parlour with her boyfriend and he was offered sex acts by one of the women working there.

"Unfortunately this is a global trend," Cr McDermott said.

"Massage parlours are often fronts for brothels and the council has a responsibility to verify whether they are or not.

"The council needs to send the message that it is investigating these places, so they feel that every customer that walks in the door could be a private investigator.

"There are too many of these places proliferating in this area, we need to send them a message."

The council voted for officers to prepare a report into how massage parlours could be investigated effectively.

Cr McDermott said he will wait for the officers' report into the options available to the council, but he thought costs should not hold the council back from investigating every massage parlour in the local government area.