A 21-year-old woman fought off what is believed to be a sexually motivated attack by biting a man in Christchurch last night.

Police are investigating the incident in Waltham, which occurred between 10.15pm and 10:30pm.

The woman was attacked from behind by a man as she walked along Vienna St towards Hastings St, police said.

"It is believed the assault was sexually motivated," police added.

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During the incident the woman was able to bite her attacker's hand and believes she bit hard enough to have left a noticeable mark, police said.

The offender was described as slim, tall (between 6'2 - 6'4), with medium-length, shaggy hair. He was also wearing dark clothing.

Police wished to speak with witnesses and anyone who knows, or has seen somebody with a visible bite mark on their hand.

People with information can contact Detective Constable Chris Healey on (03) 363 7400 or anonymously via Crimestoppers on 0800 555 111.