UPDATE: AUTHORITIES have advised residents to leave now if they plan to evacuate before tomorrow's scorching temperatures.

At a press conference this afternoon Premier Ted Baillieu, Victoria Police Chief Commissioner Ken Lay and Fire Services Commissioner Craig Lapsley urged residents to make their decision now.

"Grass is dry, bushlands are dry, it takes nothing for a fire to take control, and we've seen this in Victoria over the past number of weeks,'' Mr Lapsley said.

"Today and yesterday we've seen fires occur at 3pm and 4pm that have been difficult to control - that will only be worse with the conditions tomorrow and Friday."

Mr Lapsley said it was key to take into account that overnight temperatures would remain high.

"Last week we saw a fire burn into Seaton at 6am, that's not normal, but we will see the same conditions across Victoria Thursday night and Friday morning where the temperatures will be very hot and there will be a south westerly change moving through overnight,'' he said.

Mr Lapsley recommended every Victorian stay alert, stay connected, make a decision on whether to stay or go and have a plan.

"If that plan is to leave early then make that decision now,'' he said.

Mr Lapsley said the fire that began at Harrietville 24 hours ago was now more than 1000ha in size and the Gippsland fire had now burned more than 60,000ha.

He said the fire had the potential to travel three different ways over the next few days if it does break containment lines.

"If it starts on Thursday with a north easterly wind the fire could move towards the Toongabbie side," Mr Lapsley said.

"If later in the day the north westerly is strong it would threaten the Maffra district, and the small communities around it.

"If there is a south-easterly change, depending on where the fire has travelled, it will threaten and travel back into the bush."

He said people in the Heyfield Maffra area very much need to be aware of the changing conditions.

He also issued a warning for north western Victoria, central Victoria (including Castlemaine and Maryborough) and south western Victoria down to Warrnambool. 

Police Chief Commission Ken Lay said arson was a serious concern for authorities.

Mr Lay said he would have every available officer patrolling high risk areas in an attempt to catch arsonists.

"In the last month we suspect about 150 fires have been deliberately lit, we have charged 24 people with arson offences, half of which have been under the age of 18,'' Mr Lay said.

"The community needs to be assured that we have a large number of police patrolling problem areas where we believe fires may be lit, and we are working hard to target members of the community we believe may light fires.

"But we need the help of the community to prevent this offences occurring, we simply cannot do it by ourselves.

"We have a large number of police patrolling but we cannot stamp this out on our own, we need the help of the community and of parents.

"It is parents' responsibility to make sure children are not left alone and do not light fires.

"People need to call 000 or Crime Stoppers if they see any suspicious activity at all.''