Country Fire Authority operations manager for region six Malcolm Fallon said it was still unclear if an electrical fault or overheating was the cause but confirmed the fire originated from the ground floor file server.

The two-storey building on Gellibrand Street was home to the Department of Sustainability and Environment, Parks Victoria and the Department of Primary Industries.

Mr Fallon said firefighters were called to the scene about 11.25pm Tuesday.

``When the units arrived there was smoke coming from the top floor, out the windows and the roof,'' he said.

Mr Fallon said it took about 45 firefighters in nine trucks an hour and a quarter to get the blaze under control using an aerial appliance from Geelong.

The fire was contained to the ground floor but the upper level was damaged by heat and smoke.

A statement issued on behalf of the three government departments said alternative offices were being sought for the 45 staff, and while some disruptions were expected in the short-term, normal services would resume as soon as possible.

The Gellibrand Street building was also used as a control centre to manage major blazes during the fire danger season. Mr Fallon said he expected Colac fire station would be used as an alternative control centre.

He also said the CFA's Colac station would offer some of its available office space to the three government departments.

Any inquiries to the Department of Sustainability and Environment or the Department of Primary Industries should be directed to the customer call centre on 13 61 86. Calls to the Colac office switchboard will be diverted to this number. Parks Victoria can be contacted on 13 19 63.