Police in Christchurch have begun an investigation after a woman reported being sexually assaulted at a local park last month.

The 20-year-old had been walking through the Merrington Crescent Reserve, in Aranui, at about 10pm on Wednesday, January 6, when she says she was attacked.

The park has a pathway which connects Merrington Crescent and Breezes Rd -- the entrance being near a local church.

The woman described her attacker as a 30-year-old man of Polynesian descent. He was wearing a hoodie sweatshirt.

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Police are calling on members of the public for any information which may help identify him.

•Anyone with info can call: Christchurch Police Station on (03) 363 7400 or CrimeStoppers: 0800 555 111.