A 16-year-old prostitute was picked up in Manurewa and raped at knifepoint over the weekend, police have said.

The case comes following a weekend sting in an area notorious for underage prostitution where police say even teens not working as sex-workers are being approached by men cruising for sex.

Counties Manukau central acting crime manager Inspector Richard Wilkie said police were investigating allegations the teenager was forced to have sex at knifepoint after being picked up in Northcrest on Sunday and driven to Sykes Rd.

Detective Inspector Mark Gutry said she did not suffer serious physical injuries.

Police had a description of the offender's vehicle and were following strong leads.

Gutry said such incidents were not common but did happen.

Police are trying to deter underage prostitutes from working around Northcrest and Southmall for their own safety. Prostitutes must be 18 to legally work in New Zealand.

"Look what happens on the Sunday, one of them gets taken away at knife point.

"The group we're targeting is the young, underage people that are coming out and thinking this could be the life for them. We're going to say, no, this isn't the life for you and try and put them into care and back out doing something other than this."

Police, Child Youth and Family, the Prostitute's Collective and the local business association were involved in the weekend's operation - the second in the past month.

As a result, Wilkie said another suspected underage prostitute, also 16, was taken off the street and placed in Child Youth and Family care.

Two sex workers were arrested for breach of bail and trespass, while five unaccompanied youths aged 13 to 15 were taken from an internet café late at night and placed in CYF care.

Wilkie said teens who were not prostitutes were being approached by those seeking them, and although there was no evidence any had been picked up yet it was  "only one step away".

"We don't want them getting picked up in the wrong area at the wrong time by the wrong people," he said.

"It's a whole community approach and really it's not to try and get the kids scared of us its just to try and point out to them the risk that they put themselves at being out at that time of night. Some of them are prostituting but a lot of them are out just hanging around."

The Prostitutes Collective was involved to try and convince sex workers there were safer ways to ply their trade, Wilkie said.

"To prevent this we get them off the street, we get them educated and we get them away from that area. It's small steps but we're going to get there."

Wilkie said they were having some success and there were fewer younger people selling themselves compared with last year, particularly during the Rugby World Cup.

The presence of authorities would also drive away their customers.

- © Fairfax NZ News