A woman has accused "at least one" of New Zealand's under-20 rugby World Cup players of rape, a South African police spokesman has said.

Col. Vishnu Naidoo told The Associated Press that the 22-year-old woman said she was raped at a Cape Town hotel on Friday night or early Saturday morning, hours after the defending champion Baby Blacks lost to host South Africa in the final of the junior World Cup at the city's Newlands stadium.

Naidoo said media reports of a gang rape involving four New Zealand players were speculation and "never suggested" by police, but the woman had problems recalling details of the incident.

"No one can say whom she's alleged to be raped by at this stage," Naidoo said, adding police had conducted interviews and would rely on ongoing forensic investigations before handing the case over to a prosecutor, who would decide whether to pursue the accusations. No charges have been laid.

The New Zealand squad returned home over the weekend after losing its junior world title in a 22-16 defeat to the host country on Friday night.

"The (New Zealand) management and team have been fully cooperative," Naidoo said.

Naidoo wouldn't say if the alleged rape took place at the team hotel but South African media reported four players were involved in the incident at the New Zealand hotel in the suburb of Newlands, minutes from the rugby stadium.

"We are investigating a rape case," Naidoo said. "We have conducted preliminary investigations and interviewed various people and on completion we will forward to a prosecutor."

The chief executive of the New Zealand Rugby Union, Steve Tew, said his union would fully co-operate with the South African police investigation.

"An allegation of rape is as serious as it comes," Tew told National Radio. "The team, both players and management, co-operated fully with the authorities in South Africa.

"We did not leave South Africa until we had their permission to do so and if there's any more assistance they require, then we'll continue to co-operate fully.

"This is an incredibly series matter, both for the alleged victim and for the alleged perpetrator."

Tew said at least one member of the New Zealand team had provided DNA samples to assist the police investigation.