Victoria bushfire
© 9NEWSThe Wandin North bushfire in Victoria, Australia
Victoria will roast under temperatures tipped to soar as high as 45 degrees in the state's north-west on Saturday, with Melbourne not expecting a reprieve from the dangerous heatwave until Sunday afternoon.

"Extreme" fire danger ratings have now been applied to four Victorian fire districts for Saturday, Central - which includes Melbourne and surrounds - North Central, Wimmera and South West. A further four districts are facing "Severe" fire danger conditions.

A total fire ban has been declared for the whole weekend and a number of parks have been closed. Ambulance Victoria meanwhile is bracing for what could almost be a "perfect storm" on Saturday night, due to the extreme heat and the potential for people to party too hard.

Higher wind speeds than were earlier expected, which could gust to 70 or 80km/h on Saturday and Sunday, have increased fire danger ratings.

Melbourne is expecting a staggering 41 degrees on Saturday - an unusual peak before Christmas - with northerly winds in the morning of 35 to 50km/h. The hottest temperature for Saturday, 45 degrees, is forecast for in the Mallee including the small towns of Werrimull and Underbool.

Emergency Management Commissioner Craig Lapsley confirmed the conditions this weekend would be the worst so far this fire season. Saturday would be "extremely hot" and provide the most challenging.

"The most serious part for fire danger is actually tomorrow (Saturday) afternoon. The winds will be strong, it'll be the hottest part of the weekend. And in most parts of the state it will be reaching into the 40s," he said.

"We've had a baking period across Victoria where the fuels are now as dry as they can be in most parts of the state. And we've now got hot conditions running over a number of days and the winds will be 40 to 50 kilometres (per hour), gusting to 60 and 70 kilometres."

Paul Holman from Ambulance Victoria urged people to keep hydrated. "Heat kills because we don't drink enough water, heat kills because we don't respect it and heat kills because we complacently leave our children in cars. So make sure we heed this weather, we are going into an extreme period," he said.

"Tomorrow night is nearly a perfect storm for ambulance. We've got every available resource across the state out. Combine the heat and combine the partying and we really are going to be busy," he said.

Racing Victoria has cancelled thoroughbred racing. "The weather across Victoria tomorrow will be extreme and we have taken a view, after examining the latest forecast, that it is in the best interests of all that we not conduct any race meetings," Chief Executive Bernard Saundry said.

Tony Bannister from the Bureau of Meteorology said it was unusual for Melbourne to exceed 40 degrees in December. "This is usually a January/February thing."

He also said: "We're forecasting a record overnight min (minimum temperature) on the Saturday night into the Sunday. At the moment we're forecasting 28, the previous overnight record was 27.4 for Melbourne, for December. And it's highly unusual that we forecast records." Melbourne's average overnight temperature in December is 13 degrees.

By mid-afternoon on Friday 100 grass or bushfires had been reported in Victoria. The temperature on Christmas Day is now forecast to be 31 degrees.