Image copyright AFP Image caption Prime Minister John Key said the group's alleged activities were "abhorrent"

New Zealand's government has ordered a probe into police handling of an investigation into a group of teenagers who allegedly raped underage girls.

The male teenagers called themselves the Roast Busters and reportedly got young girls drunk, had sex with them, and posted their exploits online.

Police initially said they investigated the group for two years, but could not prosecute without a formal complaint.

However, it since emerged that a girl had made a formal complaint in 2011.

Three other alleged victims had also spoken to the police since 2011, reports said.

Prime Minister John Key criticised police handling of the case, describing it as "disappointing", and "frankly, not good enough".

"This is a very serious issue. Parents around New Zealand will want to know that in the event their daughters make a complaint to police it's taken seriously, and actually, we're entitled to know all of the facts up front," he said.

'No evidence'

Police Minister Anne Tolley said she had asked the Independent Police Conduct Authority to review how the police had handled the case.

Both she and the police commissioner had "learnt more about the case through the media than police", she said.

The alleged victim, who is now 15 years old, told New Zealand broadcaster 3News on Wednesday that she had made a complaint in 2011.

"They said that I didn't have enough evidence to show, because I went out in clothes that was pretty much asking for it," the girl said.

She added that she had given evidence in a video interview as part of her complaint.

The local district police commander, Bill Searle, said there had been "miscommunication".

"I was briefed that there were no complaints," he said. "But when I saw the footage on TV, to me that was a formal complaint. That's when I made the decision that it was to be classed as a formal complaint.

He apologised to the girl, and said that since the case of the Roast Busters was revealed by media this week, two members of the Auckland-based group had spoken to the police.

Based on their interviews, police had executed a search warrant on one address and "seized some evidence" that could help the investigation, he added.