Homicide detectives forced to use dental records to try and identify body found in burnt out car in Wandong, north of Melbourne

A body found in a burnt-out car next to a freeway north of Melbourne is so badly burned that homicide detectives are relying on dental records to identify the victim.

But police believe the body belongs to the last known person to use the car.

The body was found in the front passenger seat of the vehicle after firefighters put out the fire just off the Hume Freeway in Wandong about 4am on Friday.

"We're not sure at this stage if we're dealing with a homicide, or a suicide, or an accidental death," homicide squad Detective Senior Sergeant Stuart Bailey told reporters.

"Due to the nature of the fire ... and the injuries that occurred, we are unable to identify who that person is."

The car fire was spotted by a courier who was driving along Hume Freeway about 3.40am.

The car belongs to a 23-year-old Mernda woman who told police she lent her car to her friend, a 33-year-old Mernda man, on Thursday evening.

"At this stage we have been unable to locate that man and we suspect he may be the deceased in the vehicle," Snr Sgt Bailey said.

"We have contacted family members who are obviously extremely distraught."

It's unclear what caused the fire, but an arson chemist has examined the charred car to establish if it was deliberately lit.