The four English rugby players caught up in rape and sexual assault allegations in New Zealand are free to leave the country, police say.

New Zealand police have confirmed they are investigating "serious" allegations, but no formal complaint has been laid and the four unidentified players have refused to be interviewed after taking legal advice.

Police spokeswoman Noreen Hegarty said there was no reason to detain or restrict the movements of the players, the New Zealand Herald reported.

"Certainly if some time down the track there is some formal statement made, there would be another approach from police to speak to the people at the centre of the complaint," Hegarty said.

The players are said to have been involved in an alleged incident at the team's luxury waterfront Hilton Hotel in Auckland last Saturday night after the All Blacks beat England 37-20 in the first rugby Test.

Auckland detectives this week flew to the southern city of Christchurch where the England team are preparing for Saturday's second Test, but said the players had refused to be interviewed.

The England team is due to leave New Zealand on Sunday.

Their team management issued a statement saying the players had been advised not to answer any police questions in the absence of a formal complaint.

"The players continue to deny the allegations and have the full support of the England management, coaches and players," the statement said.

"Those players have each received legal advice from lawyers engaged to independently consider their interests.

"In the absence of any formal complaint in this matter the players were advised to decline the request made to them to voluntarily attend upon the police and they are fully within their rights to do so."

British media reports, quoting an England rugby source, claimed an 18-year-old woman had consensual sex with one player in his room before being joined by another squad member, with whom she also had consensual sex.

The reports said some of the sex was witnessed by two other players.

Hegarty said that while the woman who made the allegations had not made a formal statement, a complaint could be made at any time and would be followed up by police.

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