Wellington police are investigating a sexual assault of a young woman in the central city suburb of Mt Cook early yesterday.

A man approached the woman between 6am and 6.30am on Bidwell Street and asked her for sex, Detective Constable Leanne Thomson of Wellington police said.

After the woman - in her early 20s - turned the man down he assaulted her and tried to prevent her from getting help, Ms Thomson said.

The man was described as a dark skinned Caucasian between 18 and 25 years-old, about 1.68m tall and of slim build.

He was wearing blue denim jeans and a light coloured cloth jacket.