The government is defending New Zealand's reputation in the face of a report which says girls are forced into prostitution and foreign workers are subjected to forced labour on fishing boats.

The US State Department annual Trafficking in Persons report was released by Secretary of State Hillary Clinton and says New Zealand doesn't have comprehensive anti-trafficking laws or criminalise all forms of forced labour.

Mrs Clinton called for trafficking "to be labelled for what it is, slavery".

Immigration Minister Nathan Guy says the report covers 180 countries.

"You have to look at in its entirety," he told reporters.

"We line up with countries like Australia, Canada, the US and the UK - the overall tone of the report, I thought, was pretty good."

The report says a small number of girls and boys, often of Maori or Pacific Island descent, are trafficked domestically to engage in street prostitution while some are victims of gang-controlled trafficking rings.

Mr Guy says anyone who know anything about sex exploitation should go to the police.

"I find this terrible, people shouldn't be in fear of coming forward, if they go to the police there will be investigations."

The government has already reacted to human rights abuses and low wages on foreign-flagged fishing boats and they are going to have to be re-flagged as New Zealand ships, he says.

"Currently these boats operate under their own country's laws, with re-flagging they will operate under New Zealand law - there's a big change coming up."

Operators have been given four years to re-flag their ships but Mr Guy says they're already starting to comply with New Zealand laws.