CFA crews responded to an out-of-control grass fire near the Princes Highway

UPDATE, Thursday 8.45am: THE cause of a 5.5 hectare scrub fire in rough terrain west of Pirron Yallock is being treated as suspicious.

Detective Senior Constable Mick Palmer, of the  Colac police crime investigation unit, said the fire happened just west of the old Koala Motel site in the Stony Rises.

"The cause is being treating as suspicious and we are looking into it in coordination with a Country Fire Authority fire investigator and Parks Victoria officials," he said.

"The fire is believed to have started on private property, there was no structure damage although some minor fencing was damaged," he said.

If police can determine a cause, someone may be charged with offences.

Earlier, Wednesday night: 

A bushfire at Pirron Yallock has been brought under control.

Speed restrictions apply along the affected fire area of the Princes Highway.  

Smoke will be visible from the Princes Highway and surrounding areas and firefighting aircraft and appliances will be operating in the area.

UPDATE: 4.50pm

The fire is not yet under control.

Colac Police Sergeant Tim Kerrison said police had been on the scene early to assist with traffic management.

He said the CFA had done a good job containing the fire and said air support was particularly crucial in rocky country.

“That rocky country can be very inaccessible for vehicle units so we’re very lucky to have those airborne units to assist,” Sergeant Kerrison said.

He said investigators had deemed the fire was not suspicious and it was believed to have started from rubbish being burnt in a steel drum.

“The fire has taken off from around the base of that drum and lit up very dry grass surrounding it and spread really quickly,” he said.

Sergeant Kerrison said it was still a fire danger period and people should seek permission and use common sense.

“We are in high fire danger conditions,”he said.

“It’s been very dry and we’ve had no rain and there’s lots of dry long grass on roadsides and in paddocks.”

UPDATE: 2.45pm

The fire is still burning out of control, with 20 firefighting vehicles and a firefighting helicopter working to control the blaze.

A CFA spokesman said the two-hectare fire was in difficult terrain posing access challenges.

He said the fire was fuel-driven rather than wind-driven and was slowly moving in a south-easterly direction towards Pirron Yallock and Swan Marsh.

The spokesman said the fire had not impacted on any people or properties but there were concerns it was moving towards plantation assets.

EARLIER: Emergency services have been dispatched to tackle an out-of-control grass and scrub fire close to the Princes Highway at Pirron Yallock, near Colac.

More than twenty vehicles are responding to the blaze, which is travelling in a south-easterly direction towards Pirron Yallock and Swan Marsh.

A watch and act warning has been issued, with emergency services warning the safest option is to leave now because conditions may change and get worse very quickly.

A CFA spokeswoman said the incident was reported just after 11am, with seven firefighting vehicles at the scene and seven more on the way.

A firefighting helicopter was tackling the blaze, and the CFA spokeswoman said police had been requested to attend as well.

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More to come.