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A man is being sought after a woman was thrown into a car boot in Christchurch in what police say was a "totally unprovoked" incident.

Detective Ashley Ross said the 29-year-old student was walking along Barlow Street from Creyke Road in Ilam about 6.35pm on Monday when a young man asked her to help start his car.

As she went to help, he pushed her into the open boot.

"She kicked open the boot before he could close it. She managed to force her way out of the boot and escape, alerting a passerby," Mr Ross told The Press.

"It was totally unprovoked and we obviously don't know what his intentions were."

The woman was upset and could not believe what had happened.

Police were concerned someone else could be targeted.

The offender was described as a Maori or Pacific Island teenager, 1.65m tall, with black, frizzy hair and of a thin build. He was driving a white, five-door Japanese hatchback.