Sergeant Simone Prictor, of Ararat, said police and Country Fire Authority crews attended the fire about 3.45am.

The tree concerned is on the northern side of the highway about 200 metres west of Hillside Road.

The investigation into the cause of the fire is ongoing, Sergeant Prictor said.

"It appears to have been deliberately lit," Sergeant Prictor said.

"One lane remains closed. People need to be mindful of that and slow down - we don't want people just flying through there."

VicRoads personnel will assess the tree for safety and determine if it can remain standing or is too unsafe.

Once that assessment is made, police will re-open the road.

Detective Senior Constable Gjalt Erkelens was at the scene throughout the morning and said he did not believe the tree was under threat from the Western Highway duplication project.

A Western Highway duplication project protest camp is located nearby, but Sergeant Prictor said police did not believe any activists were involved in the fire.

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