A large blaze at a vitamin supplements factory in Melbourne was deliberately lit, with police now searching for a man spotted fleeing the building with his shoe on fire.

The Boxing Day blaze gutted the building in the city's south and saw at least 50 firefighters battle for hours to prevent it from spreading to nearby buildings.

Police have revealed that the blaze is suspicious as a man was seen entering the building just after 4am (AEDT).

The building's CCTV footage shows the man in a dark hoodie and track pants walk towards the building while carrying a large red petrol container.

A minute later, another camera inside the building shows a burst of smoke and flames.

The man emerges from the smoke with his left leg engulfed in flames.

He stomps his feet as the fire lick at his ankles before running out of the building with his left shoe still burning.

A police spokesman said detectives are hopeful someone may recognise the man from the footage as he likely suffered burns to his body.

Anyone with information about the fire in the Dingley Village industrial area on Thursday is asked to call Crime Stoppers.