A man has been arrested and another is fighting for his life following an alleged assault in Victoria's northwest.

A 59-year-old man is in police custody and a second man believed to be in his 60s has been flown to hospital with life-threatening injuries following an altercation in Wheatsheaf near Daylesford about 10.30am on Tuesday.

Ambulance Victoria confirmed they treated a man at the scene and a police investigation is ongoing.

"Paramedics treated a man believed to be in his 60s for serious upper body injuries," Ambulance Victoria said in a statement.

The man is said to be in a critical condition.

The Country Fire Authority also responded to the scene after a fire broke out.

"Crews extinguished a small patch of fire impacting forest area," a spokesman said.

Police said their investigation into the incident was ongoing and anyone with information was urged to contact Crime Stoppers.