A woman was saved from a brutal late-night sex attack at a Christchurch park when a passing street-sweeper scared off her attacker.

The woman in her 20s had stopped her car at Nunweek Park about 2.30am to use the public toilet.

After finding it locked, she was attacked from behind, pushed to the ground, head-butted and hit in the face.

"While on the ground her attacker has attempted to sexually assault the woman, but was disturbed by a passing street-sweeping machine," said Detective Sergeant Richard Quested today.

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"The woman clawed at the man's face and pushed him off, and was able to run back to her car and drive away.

"She has suffered cuts and bruises but fortunately was not seriously injured."

The offender is described as being between approximately 167cm (5ft 6in) and 177 (5ft 10in) tall, of medium build with a 'beer belly' and a moustache. He was believed to be wearing a black cap with a black hoodie pulled up over the cap.

Police are now appealing for any information from members of the public who may have been in the Harewood area at the time.

"We would like to hear from anyone who was driving through or in the vicinity of Nunweek Park between 2 and 3am on Monday morning," Mr Quested said.

"If anyone has seen a man of this description, or any other suspicious people or activity around the park, we urge them to contact Police as soon as possible."

Anyone with information is asked to contact Constable Pamela Cox on 03 372 5710, Christchurch police on 03 363 7400, or anonymously through Crimestoppers on 0800 555 111.