'Concerning behaviour': Woman sexually assaulted while running in Victorian park

Police are concerned and on alert after a woman was sexually assaulted while running in a park north-west of Melbourne.

The woman was going underneath a bridge in Jacana Reserve, near Johnstone Street, about 11.30am on August 14 when she heard someone come up behind her.

Sexual crime squad detective inspector Mark Burnett said she was thrown to the ground by a male stranger and sexually assaulted.

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He said the woman fought and yelled for help before she managed to get away and ask two passers-by for help, who then dialled triple-zero.

The woman was taken to hospital with minor injuries, including scratches and pain in her shoulders.

A major search — including police, a helicopter and the dog squad — was launched in an effort to find the man but he was unable to be found.

"At this stage we believe this was an isolated incident and we don't believe it's connected to any further incidents," Burnett said at a press conference today.

"This behaviour is definitely concerning for police and the wider community."

Burnett said the victim recalled the man wearing a dark jumper and pants at the time.

CCTV obtained by police showed the man running towards the woman before the sexual assault.

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Police are patrolling the area and have shared CCTV and images of a man they wish to speak with.

He is believed to be in the area at the time of the incident and is urged to come forward.

Police are also keen to speak with anyone who may have witnessed the incident or seen any suspicious activity.

Anyone who lives or was in the area near the Jacana Reserve between 9.30am and 12.30pm on August 14 and may have CCTV or dash cam footage is urged to come forward and contact Crime Stoppers on 1800 333 000.

Support is available from the National Sexual Assault, Domestic Family Violence Counselling Service at 1800RESPECT (1800 737 732).