The fire was declared under control about 7.40pm after a large firefighting response.

However, the mountain is expected to be closed for a number of days as fire crews work to guarantee it is safe.

Buninyong-Mt Helen Fire Brigade Lieutenant Matt O'Brien said there was a helicopter, more than 10 tankers and about six other smaller vehicles fighting the blaze.

However, due to the difficult terrain, crews were forced to wait for much of the fire to burn out before they could access it.

It is not yet known exactly how the fire started, but it is believed to be suspicious.

"We had to wait up the mountain for the fire to burn out before we could access it to contain it," Lieutenant O'Brien said.

"It's burnt a fair bit of the mountain and it will probably be closed for a few days."

Although summer is still four weeks away, Lieutenant O'Brien said people still had to be cautious.

It was little more than a month ago that Mt Buninyong was covered in snow.

"We're not in the peak of summer so it was still pretty moist under foot, but there was lots of fuel," Lieutenant O'Brien said.

"It was a warm day with a bit of wind and still you see what what can happen."

EARLIER, 6PM: A fire has broken out on Mt Buninyong this afternoon.

Light, grey smoke can be seen billowing from the mountain, with the fire igniting just before 5pm.

As of 6pm, there were eight firefighting units fighting the blaze.

The fire has started on the northern side of the mountain, near Hogarths Road West.

An advice warning has been issued by Fire Rescue Victoria; the lowest grade of warning messages.

It says the fire is "not yet under control".

It says: there is currently no threat to communities, but you should continue to stay informed and monitor conditions.

Although not threatening any homes, the blaze is proving difficult to fight due to rough terrain.

A firefighting helicopter is believed to have been called just after 6pm.

As of 6pm, the temperature was still 27 degrees in Ballarat, with 18km/h northerly winds.