A cyclist was the fourth person to die on Victoria's roads on Saturday after he was hit by a truck near Portland.

The 18-year-old Portland man was riding a motorised bicycle when he was struck on the Henty Highway about 5.30pm. He died at the scene.

The truck driver, also a Portland man, is assisting police with their enquiries.

Earlier, a motorcyclist died after colliding with an SUV in Heatherton Road, Springvale South, about 4.45pm. He died on the way to hospital.

The other driver, a 35-year-old Springvale man, stopped at the scene to help.