A sex attacker has assaulted a woman in Wellington central, and is on the loose.

The woman was attacked by the stranger while walking in Brougham St, Mt Victoria, about 6.10pm on Saturday.

A man with his hood up approached the woman as she walked.

"He grabbed her and at the same time he grabbed her, he put his hand up her skirt, and then quite soon after ran away," police Senior Sergeant Simon King said.

Police were interviewing the woman to get a detailed description of the perpetrator.

"At this stage we haven't located him, but we're speaking to the victim to see what we can establish from her," King said.

Police asked the public to be careful walking in Mt Victoria.

"When walking in that area be aware of your surroundings," King said.

"If you see anyone suspicious, call for help, or find a different way to go."