Firefighters were battling in difficult conditions last night to bring under control a 450-hectare fire burning in the Stoneyford area east of Cobden.
There was speculation about the cause of the outbreak, with the CFA confirming it was believed to have started from two locations.
A Camperdown resident said there two separate plumes of smoke in the area when he saw the blazes about 4.30pm from the top of Mount Leura.
However, CFA information officer Peter Hamilton declined to say whether the cause was believed to be suspicious.
More than 30 CFA units and a number of aircraft attacked the grass fire that was reported shortly before 3pm yesterday near Scoullers Road at Stoneyford, west of Colac.
A stiff south-easterly wind quickly pushed the blaze westwards across farmland.
At one stage it threatened three houses at the corner of Scoullers Road and the Cobden-Stoneyford Road but firefighters were able to protect them.
The fire travelled south of the Cobden-Stoneyford Road and crossed Ansons Road before crews were able to slow its progress as winds eased before 8pm.
Mr Hamilton said aircraft were used to attack the head of the fire.
Mr Hamilton said earthmoving equipment was brought in last night to cut control lines on the southern flank between Scoullers and Ansons roads.
Crews were expected to work through the night to bring the fire under control.
The blaze closed the Stoneyford-Cobden Road between the Princes Highway at Stoneyford and Cobden.
The road remained closed at 9pm last night.
The fire came as the south-west remains on alert ahead of several days of high temperatures.