Cause of fire that destroyed historic century-old Bannockburn church unknown

A 140-year-old church near Geelong, southwest of Melbourne, has been destroyed in a ferocious fire overnight.

Firefighters were called to the blaze at St John The Evangelist Catholic Church on High Street in Bannockburn just after 4am and arrived to find the weatherboard church completely engulfed in flames.

"Some of the parish members have gone past already and expressed some concern," CFA spokesman Barry Planner said.

As well as the church itself, valuable items housed inside the building have also been completely destroyed.

Fire investigators earlier told 9NEWS the blaze was not believed to be suspiciously lit, however an investigation will be undertaken to determine the cause.

There were four Catholic churches set on fire around Victoria in April, all connected to notorious pedophile priests, including Kevin O'Donnell and Ronald Dennis Pickering.