A Melbourne father has made a tearful appeal for a man who rescued his daughter as she was being sexually assaulted in a park on Thursday afternoon to come forward.

Key points: A good Samaritan has pulled an attacker away from an 18-year-old victim

A good Samaritan has pulled an attacker away from an 18-year-old victim The woman was being sexually assaulted in a park when the man intervened to save her

The woman was being sexually assaulted in a park when the man intervened to save her Her father has made a tearful appeal for his daughter's rescuer to come forward

The father said he wanted to thank the good Samaritan for stopping the attacker who was trying to drag his daughter into bushland at the park in Melbourne's north.

Police said the rescuer was walking with a woman and an infant in a pram in the Malcolm Creek Park in Craigieburn when he heard the young woman's screams.

The father said the attacker could still be "lurking around" and he wanted to thank the family for saving his daughter.

"Just to say thank you for what you've done, because you ultimately saved, maybe, my daughter's life," he said.

The woman, who turned 18 last week, was walking to work at 5:50pm when she was grabbed from behind and sexually assaulted.

Her father on Friday joined police to call on the couple who intervened to come forward.

"Please contact the police, let's get this guy off the street, obviously he's out there and he may do it again," he said.

'Something bad was about to happen'

Detective senior constable Rick Barry said the man who intervened had physically pulled the attacker away from the young woman.

"The attacker was trying to pull her further into some parkland and the passerby was able to intervene and get the man off her."

Senior Constable Barry praised the passerby for his bravery.

"There's no telling what would have happened if he hadn't intervened," he said.

"People do throw the word hero around, but this man did say 'I need to do something as part of a community, a community looks after itself'."

The rescuer said his daughter had at first thought the attacker was a friend trying to cuddle her.

"As she's turned around she's just panicked, shock, terrified, then … realised that something bad was about to happen," Senior Constable Barry said.

"She just fought him off as best as she could until this family intervened."

The family walked the woman to the nearest road, but they did not exchange details.

The woman's father said his daughter was usually a "bright, bubbly, very funny" person, but he said now she was quiet, withdrawn and thinking of the "what ifs".

The attacker was wearing a black baseball cap and a white t-shirt.

He is described as aged in his 20s, Caucasian, with dirty brown-blond hair and a thin build and was about 183 centimetres tall.