It was raining sheep on Melbourne motorists after a truck carrying 400 livestock rolled off a bend and hung precariously over a highway.

The dramatic crash in Laverton North, in Melbourne's west, occurred about 9pm (AEST) as the sheep truck was travelling to Geelong on the Western Ring Rd off-ramp, police said.

Dead and injured sheep fell on the Princes Highway overpass below, hitting two vehicles.

One of the cars rolled numerous times but the occupants were not injured, police said.

The truck driver is also uninjured.

The inbound Princes Highway has been blocked, with one outbound lane of the Westgate Freeway also closed.

Police said the sheep carcasses will likely be cleaned up by council rangers this morning.

No other vehicle was involved in the crash.