They ran to help the woman and her attacker fled in the direction of the wharf.
The suspect is described as a Caucasian male in his 40's.
He is of solid build, with sandy blonde, shoulder length, unkempt hair, and was clean shaven.
He was wearing a light coloured t-shirt, with a large logo or picture on the front and light blue jeans
This is the second attack on a woman in Dunedin in a week.
On Monday, a 23-year-old jogger was knocked to the ground, tied up and sexually assaulted about 9.15pm in a Dunedin park.
Anybody with information on either cases are asked to call the Dunedin police on 03 471 4800. Information can also be passed on anonymously via Crimestoppers on 0800 500 111.