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At least three men must know the identity of another man who sexually assaulted a young Australian tourist in Christchurch, police say.

The woman, 20, is recovering with friends after the attack in the early hours of Sunday.

She had left a bar in the city about 3.30am and become separated from her friends in Manchester St.

She was approached by four men, two of whom left immediately.

Another of the men left a short while later, leaving her alone with the final man, who lured her into Aberdeen St, off Manchester St.

"Once in the street, the male punched the victim to the ground and kicked her a number of times in the head, before sexually assaulting her," Detective Kelvin Holden said.

The woman fought off her attacker, who ran off carrying his pants.

Mr Holden asked the three men involved earlier to talk to the police.

"These three guys know who this offender is," he said. "It's best if they come to us before we go looking for them."

The woman was not in hospital and had not decided whether to continue her stay in New Zealand.

"She's obviously pretty shaken but she's got good support from friends so she's doing all right."

Police have described the offender as 18 or 19, a dark-skinned Pakeha or possibly part Maori, with eyes giving a slight Asian appearance.

He is 185cm tall, of skinny build, and was wearing a light-brown hoodie, baggy dark pants and a headband.

Meanwhile, the hunt continues for a man who raped a 16-year-old Turangi girl in Porirua on Saturday night.

The victim was drinking with friends near the Cannons Creek shops about 11.30pm when a man approached and took her through Cannons Creek park to an area near changing rooms at the Castor Cres end of the reserve, where he raped her in a 20-minute attack.

Detective Sergeant Andrew Saunders said police were talking to people who were around the area.

The offender is described as a muscular Polynesian, about 18 or 19, with short black curly hair.

He was wearing a blue lavalava and a brown and blue shirt with horizontal stripes, and is possibly called Jaam or Jamie.

One of the girl's friends thought he looked familiar, leading police to believe he is a Porirua resident.

The girl is recovering with relatives.