Temperatures are forecast to climb into the 40s over the next four days, and firefighters are on high alert.

Temperatures, driven by searing northerly winds, are expected to hit 41 in Melbourne today, 43 in Mildura and 44 in Horsham.

The heatwave is expected to be the worst since 1908 and follows a record 12-year drought.

In South Australia, the heatwave is set to continue, too, with the temperature reaching 44deg in Adelaide and 40deg in many regional centres.

In Victoria, a total fire ban has been declared today across most of the state and about 100,000 volunteer and staff firefighters are on standby.

A chance of storm in the ranges poses the risk of fires sparked by lightning.

Premier John Brumby yesterday warned that Victorians are "facing an unprecedented week for our state" with temperatures expected to be in the low 40s for the rest of the week.

He said many parts of the state face hot and arduous conditions with a zero prospect of rain.

"This means it will be a difficult period for us in many ways," Mr Brumby said.

Mr Brumby reassured Victorians that emergency services have taken steps to ensure they are properly prepared.

There should be adequate power for the week and electricity companies have put on extra maintenance staff to deal with any problems arising from extra demand, he said.

The Premier said the most important thing is for regional families to have a fire plan.

Victoria's Emergency Services Co-ordinator Bruce Esplin said emergency services are on high alert and ready for a bushfire scenario similar to that faced in 1983.