Detective Sergeant John Hedges, of Dunedin, said the woman was walking home from the centre of town when she spoke to an unknown man, believed to be aged about 20, who lent her his sweatshirt.
A short time later she was sexually assaulted, suffering chipped teeth and facial injuries in the attack.
Police would like to talk to anybody who saw the victim walking north on the seaward side of George St about 4.30am.
She was 1.6m tall, wearing a bright pink knee-length dress and black thigh-length coat, and had long blonde hair.