A blaze that threatened homes in Melbourne's south-east on Wednesday is now being treated as suspicious.

About 40 homes and businesses in several streets in Cheltenham were evacuated after a grass and scrub fire took hold in Cheltenham Park about 2.15pm.

Victoria Police and the Metropolitan Fire Brigade confirmed the blaze was being treated as suspicious.

Earlier reports said it had originated from an out-of-control campfire.

Detective Senior Constable Andrew Howell told reporters on Thursday that police believed the fire had been lit in a campfire area of the park but were still trying to determine what caused it to spread.