"I won't be going out at night for a while."

He warned others to watch what they were drinking and "definitely be careful" about who they befriended.

"Up until they pulled out the knife, they were extremely friendly," he said.

Warrnambool police are now urgently seeking witnesses to the incident.

Detective Senior Constable Colin Ryan, of the Warrnambool Criminal Investigation Unit, said the man gave police a description of the couple.

The male offender was described as being in his early 20s.

The woman, described as very thin, is believed to be in her late teens with blonde hair in a ponytail.

Detective Senior Constable Ryan said any witnesses were asked to contact the Warrnambool police station or Crime Stoppers on 1800 333 000.

"It was obviously a very frightening experience," he said.