Firefighters have stumbled upon a giant cannabis crop in a Springvale factory fire overnight.

Country Fire Authority firefighters forced entry to the Rosalie Street building after passers-by saw smoke coming from the factory about 11.30pm.

Once inside, they were confronted with an extensive hydroponic set up, where hundreds of semi-mature cannabis plants were growing.

It is believed the blaze was caused by a electrical fault. No one was inside the factory at the time.

Emergency crews at the factory in Springvale on Sunday night.

A crime-scene guard has been posted outside the factory and authorities will now investigate who was renting the building.

Anyone with information is urged to contact Crime Stoppers on 1800 333 000.