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He made the comments on Radio Live when asked about the Chow Brothers' bid to build a 15-storey brothel and hotel building in the Auckland CBD.
Mr Key said he didn't think the Prostitution Law Reform Act 2003 had achieved a reduction in street prostitution and underage prostitutes.
"The argument was that it would eliminate all the street workers and underage people, particularly girls, and the reports that we see in places like South Auckland is that it hasn't actually worked.
"I think it's been marginally successful, if at all," he said on Radio Live.
Despite his comments on Radio Live Mr Key, said he didn't have an easy answer for solving the issues of street prostitution and youth under 18 engaging in it.
Mr Key said it was not a top priority issue for the Government to address during its current term.
He didn't think there was political will to revisit the legislation, but said there was support for looking at street prostitution.
Mr Key said it was better to have prostitution occurring in licensed premises, such at the proposed Chow Brothers' brothel and hotel, than next to a school or other residential areas.
Opponents of the venue are labelling it New Zealand's first sex hotel.