Police are worried by a rise in underage prostitution in downtown Auckland, where girls as young as 12 are selling themselves for sex.

Senior Constable Mark Riddell of the Auckland central police youth action team said that in the past six weeks operation City Door had identified at least 13 girls aged under 16 as active prostitutes.

Many of them work from City Rd, which runs between Queen St and Symonds St.

Mr Riddell calls the street a "young red light area".

In the past two weeks, Mr Riddell and his team have taken five underage girls off the streets and put them into Child, Youth and Family custody.

But he said many of the girls escaped from CYF and went straight back on the street.

"Kids will run away on the same night we pick them up. On some occasions, they've got back to the city before we've got back out on the road." One of the girls, who started working as a prostitute when she was 12 and had never been to high school, said she had been picked up by a car full of men and raped only days earlier.

Police placed the girl in the care of CYF, but Mr Riddell said she ran away soon after.

CYF had only about 100 beds in secure custody. Those at risk of suicide were given priority so the girls he dealt with often missed out.

Debbie Baker, the manager of Streetreach, a group supporting street sex workers, said she knew of at least 12 girls between 11 and 15 "out there selling themselves for sex" in the central city.

"Young meat earns a lot of money," said Ms Baker. "Underage prostitution has always been a problem, but there is an increase.

"We're seeing more and more young girls out there."

Ms Baker said she knew of a 12-year-old West Auckland girl who was recruited by a gang outside her school to sell cannabis.

After spending the drug money, the girl was forced into prostitution to pay her debt to the gang and she shared the extra earnings from her work with her family.

"Her parents knew exactly what she was doing."

Ms Baker said she believed the police were under-resourced and CYF was unable to deal with the problem.

"These girls may be abused in their home and all the police have got the power to do is to take them back there. The police really do have their hands tied.

"We need to protect our children - and these are children, regardless of where they've come from.

"Child prostitution is child abuse and it needs to be given the same penalties. It needs to be given the same priority as child abuse."

Auckland Mayor John Banks said he was concerned by what he called an epidemic of prostitution, family violence and drug and alcohol abuse in greater Auckland.

"We'll never be a Super City as long as we've got kids as young as 12 on the streets selling themselves for sex."