A calculated arsonist behind a fire burning for weeks in a Victorian coal mine will likely strike again, police fear.

The firebug experimented with test fires before deliberately setting three spots alight to create a large blaze that spread into the Hazelwood open-cut mine on February 9, as Victoria faced its worst bushfire conditions since Black Saturday in 2009.

Police saturated the area in the Latrobe Valley on February 9 but were unable to stop the arsonist.

Chief Commissioner Ken Lay said the arsonist would likely strike again.

“This person has shown absolute and total disregard for life and property,” Mr Lay said.

“This person took actions which were very, very calculated and designed to create significant damage to the community.

“It’s very, very worrying that this person is still in our community.”

Fire Services Commissioner Craig Lapsley said the arsonist was calculated because they deliberately set fires during the hottest part of the day when a wind change was forecast.

“We’ve got at large one of the most serious parts of arson — someone who maliciously puts fire in the environment at the most critical part of the day when the weather conditions are the worst.”

Lead investigator Inspector Mark Langham said police were out in force on February 9, patrolling the areas that had been previously targeted.

“We saturated the area, put out extra patrols, so it’s extremely disappointing that someone’s still been able to light a fire that’s had this impact without us having the information and being able to stop it.”

Mr Lay said police had teams out on days of very high fire danger targeting arsonists and areas where fires may be lit.

“Unfortunately on this occasion we missed this spot but there had been an enormous amount of work done in the Morwell area at the time. We knew it was a very high-risk area.”

Police believe the arsonist is from the Latrobe Valley or wider Gippsland area and have appealed for public help.

The charge of deliberately lighting a bushfire carries a maximum jail term of 25 years.

It’s expected to take at least another fortnight to put the coal mine fire out.