A woman who was sexually assaulted in a park in Melbourne's northern suburbs fought back and escaped after grabbing the assailant's knife and stabbing him, Victoria Police have said.

Key points: Police said the woman was attacked while trying to help the man find a lost item in Mayer Park

Police said the woman was attacked while trying to help the man find a lost item in Mayer Park She was sexually assaulted, then a struggle ensued and she managed to grab his knife, they said

She was sexually assaulted, then a struggle ensued and she managed to grab his knife, they said The woman believed she had cut him and there was blood at the scene

The woman, 32, was walking a dog in Thornbury about 10:30pm, police said, when she noticed a man ahead of her acting suspiciously.

To avoid him she walked into Mayer Park.

He approached her asking for help to find something he lost.

Then, brandishing a knife, he pushed her to the ground and sexually assaulted her.

The suspect is in his early 20s with brown hair and he was wearing a hoodie and loose pants. ( Supplied: Victoria Police )

The woman managed to grab the knife in the struggle that followed and stabbed him, according to Detective Acting Inspector Janet Michell of Victoria Police.

"She believes that when she took the knife and went to defend herself that she made contact with the offender and we believe there is blood at the scene," she said.

Forensic police scoured the park for evidence after the sexual assault. ( ABC News )

The woman fled with the dog to seek help from a driver at a nearby set of traffic lights.

Flowers were left near the scene of the sexual assault in Mayer Park. ( ABC News: Justine Longmore )

The officer said the woman was defending herself and it could have ended up much worse.

"I think the woman was very brave to act the way she did. She obviously made an assessment and a decision on how she would handle it and it turned out very well for her," she said.

The woman went into the park to get away from the man who was acting suspiciously. ( ABC News: Justine Longmore )

Detective Inspector Mitchell said it was a frightening incident and the man was intent on pursuing her.

"I think he was on a mission and he saw the woman coming and he pursued her," she said.

Police released a photofit of the man they are looking for, who is described as Caucasian, in his early 20s, of medium height with brown hair.

He was wearing loose pants and a hoodie and may seek medical treatment for a stab wound.