Mallee and Northern Country districts have both been labelled as Code Red - the highest warning level.

Emergency Management commissioner Andrew Crisp said if a fire started in the conditions predicted it would be uncontrollable, unpredictable and fast-moving.

He said fire agencies would find these fires difficult to control.

There is a high likelihood that people in the path of a fire will be killed or seriously injured in these conditions.

The warning also means some schools and services will be closed.

Mr Crisp said a Code Red day was determined because a combination of high temperatures overnight, little relief followed up with predicted high temperatures and high wind speeds on Thursday.

There is a statewide total fire ban for Thursday.

Mr Crisp said the state had not experienced a Code Red day under the current framework, and it needed to be taken seriously.

"Our message to community members in these areas is that leaving high bushfire risk areas tonight or early tomorrow morning is the safest option," he said.

"We know this can be difficult to plan for and some will find it inconvenient.

"As a result of the predicted conditions, some schools and early childhood centres will be closed in some weather districts and community services may not be available.

"Your plans need to suit your circumstances, however going to your nearest regional city centre, going to the movies, the pool, or a shopping centre are some options people can consider."

Mr Crisp said it was already hot and people needed to ensure they were looking after themselves and others, particularly those who need additional assistance in the heat.

"Our community members should be prepared for fire, and know where to get information to make good decisions about your safety," he said.

"Given fires could start and move quickly, you won't always receive a warning or be told what to do if a fire starts. The fire agencies advise you not to wait and see, it's up to you to be prepared and to know what you will do.

"The safest place to be is away from high risk bushfire areas in a Code Red and leaving the night before or early in the day is the safest option."

A Code Red day means that all public parks will be closed, some community services will be unavailable and some council facilities will be closed.

During extreme weather, such as a Code Red day, localised power blackouts could occur.

People are urged to have an emergency kit ready, including medication, battery-operated torches, radio and phones.

The Emergency Management commissioner said people should:

Country Fire Authority chief officer Steve Warrington said Victoria's fire agencies and emergency managers were on high alert and prepared for the worst conditions.

"Tomorrow's conditions will be extremely difficult for firefighters and the community if we get fires running," he said.

"The reality is fires are incredibly difficult to control and there are many aspects our communities need to consider in planning for it.

"Homes are not built to withstand the types of fires we may see on a Code Red day and you don't want to be caught travelling through areas on fire at the last minute if you wait and see."

The Bureau of Meteorology forecasts other fire danger ratings based on weather conditions and fuel load.

However, a Code Red Day is determined by the Emergency Management commissioner. The commissioner takes into consideration a range of factors including fire dander ratings, fuel conditions, agricultural activity, fires already burning, uncontrolled ignitions, community risk, population movement and major events.

The commissioner makes this determination through consultation with emergency management heads including fire, health, police, transport and energy representatives.

The Code Red Day determination is linked strongly to Bureau of Meteorology forecasts of extreme fire danger rating in either grassland or forest.

This is the first time a Code Red Day has been determined by the Emergency Management Commissioner under these arrangements.