A security guard was brutally bashed and had his vehicle stolen after stopping to check if the occupants of a car involved in two separate crashes were okay.

Charlie, 49, said he was patrolling in Truganina in Melbourne's west in the early hours of Thursday morning when he saw a Volkswagen speed away from a crash that left another car smashed up.

Security guard Charlie was left with a fractured eye socket and broken nose after the assault in Truganina. Credit:Nine News

The security guard told Nine News he followed the Volkswagen until it went down an embankment at Tilwinda Place about 12.45am.

Charlie, who did not want his last name used, said he got out of his patrol car to see if they were okay. That's the last thing he remembers.