About 1000 fires have been burning across Victoria with lightning strikes starting new blazes in the popular tourist destination - Grampians National Park.
Fire crews are trying to get on top of all fire activity ahead of a potentially dangerous wind change on Friday afternoon.
Most of the 1000 fires reported in the last 24 hours were small incidents that have been handled but there are 35 main fires listed as "going" by Thursday lunchtime.
"We can hope that we get on top of those and have controlled issues around those fires as soon as possible," Fire Services Commissioner Craig Lapsley said.
"However what it's telling us is that any fire in the environment now will be absolutely critical for Friday."
The largest fire has burnt more than 6600 hectares near the town of Hopetoun in the state's northwest.
The fire is very active in large sections of the Wyperfeld National Park and crews are working to stop it affecting neighbouring private property, a State Control Centre spokeswoman said.
Lightning sparked fires at the north and south ends of the Grampians National Park, a popular tourist area, overnight Wednesday.
The spokeswoman said while there was no threat to tourists at this stage, they needed to keep updated on the fire situation.
"At the moment where the fires are going is deep into the national park, but it is an area that is frequented by a lot of people in summer," the spokeswoman said.
As well as the Grampians, lightning sparked fires in Gippsland and central Victoria.
A fire burning at Kangaroo Ground, on Melbourne's northeastern outskirts, was brought under control on Wednesday afternoon.
The worst fire conditions are expected with a southwesterly wind change on Friday which Lapsley said would push any fires in a different direction.
"Fires are running at their most intense when you have a wind change," Lapsley said.
"It's the worst scenario possible but it's a reality.
"It's forecast for tomorrow."
Lapsley said the next six weeks will be a critical part of the summer fire season.
"To have four or five days of excessive temperatures during the day and during the nights is baking Victoria. It is taking the last amount of moisture out of any of the fuels."
Some parts of Victoria face temperatures of 45C on Thursday, while Melbourne is forecast to reach 44C.
Friday will be the fourth day of the heatwave with temperatures over 40C with the wind change expected to hit Melbourne around 5pm (local time).
While Melbourne and the state's south will enjoy cooler weather over the weekend, Lapsley warns the cool change will not go through the northeast until later on Saturday.
A total fire ban has been declared for the entire state on Thursday and Friday.