An Auckland policewoman who boosted her income moonlighting as a prostitute in a top massage parlour will have earned up to $500 a night.

She worked in a "very classy brothel" that employs several attractive women and was well taken care of by management, a spokeswoman for sex workers said yesterday.

Police said the officer, who would take home at least $43,000 a year from the force, did not ask permission to take a second job but if she had, approval would not have been granted for sex work.

It is understood financial difficulties led her to find a second job, but the Police Association has condemned her choice of work.

The policewoman has not been identified but works in the Auckland City district, where female officers make up around 20 per cent, or 132, of the 660 sworn staff.

Deputy Commissioner Lyn Provost said the officer worked "for a limited time" as a prostitute before this was discovered this year.

Police would not comment on how they found out about the prostitution but said it was treated as an employment matter.

"I can assure the public that police have acted properly in this matter and that this type of secondary employment would never be approved given that the type of work is inappropriate and incompatible with policing," Mrs Provost said.

The policewoman had the support of the New Zealand Prostitutes Collective, said Auckland manager Annah Pickering. "Everyone has to make some sort of living. If she had to pay off certain debts, by all means we totally support that person."

Police Association president Greg O'Connor said there was sympathy for the situation which led to the woman's sex work.

"But it doesn't change the fact that instinctively police officers know this is not right."

Mr O'Connor said the association was "very uncomfortable" with the woman's choice of work. "It's still regarded as a twilight industry that is a major conflict of interest with what we do as police officers."

Many police officers struggled financially, especially in the major cities, and more and more sought secondary employment, Mr O'Connor said.

Ms Pickering said sex workers came from all walks of life.

"We have had law students, doctors, students, and I don't think there's anything wrong for a police officer to be a sex worker."