BENDIGO firefighter Bob Lindsay says grassfires can re-ignite in a split second.
The warning came as more than 30 firefighters spent all day Friday blacking out a major grassfire that burnt through Toolleen on Boxing Day.
More firefighters will be patrolling the area again on Saturday to monitor any flare-ups.
Mr Lindsay said crews were able to use thermal imaging cameras on the ground and in the air - from a helicopter infrared camera - to find embers and hotspots.
He said they had uncovered a number of deep-seated fires still burning beneath trees and fence posts.
“We use the camera to detect hotspots that are not normally visible to the naked eye," he said.
"We can go straight in and deal with them.
"Our mission is to secure the area and the flanks of the fire perimeter."
Mr Lindsay said an ember could catch onto unburnt ground and quickly start a fire.
"It happens all the time that a hotspot can be left burning in an old tree or fence post and can reignite. It can happen extremely quickly, within a second it can be up and flaring again."
Mr Lindsay said some hotspots at Toolleen were burning at close to 500 degrees.
The fire was sparked at 2.40pm Thursday and was declared safe yesterday as CFA and DEPI crews worked to clear burning trees from the area.
CFA incident controller Frank Oliver said the grassfire was the biggest he had seen in his 10 years as a CFA volunteer in Toolleen.
He said the fast-moving grassfire had shown "unusual behaviour" in the way it raced up hill.
"It was certainly the biggest we've had for many years," he said.
"And in terms of speed it was one of the most rapid fires we've ever had."
"It showed no sign of slowing down."
Mr Oliver said he was particularly concerned about Saturday's extreme fire conditions.
He said crews were on alert for the high risk of more fires in the region.
“Given the conditions with the wind and temperatures it's a major concern," he said.
A Total Fire Ban is in place Saturday for the Central, Northern Country and Mallee regions, with a forecast top of 38 degrees in Bendigo.
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