VICTORIAN farmers are sweltering through the worst heatwave since the Black Friday fires tore through the state in 1939.

The hot spell comes during one of the driest Januaries on record, with rainfall totals well below long-term averages and no rain forecast this week.

As temperatures soar into the 40s, fruit growers are battling to protect their crops from the searing sun.

Drought-stressed grapevines are already showing signs of heat damage, according to Murray Valley Winegrape Growers chief executive Mike Stone.

Mr Stone said sunburn was already evident on north-south planted rows.

He said growers on the Victorian side of the border, who could not afford supplementary water and are on allocations of 33 per cent, would likely run out of water before the heatwave ends.

"If you haven't been able to irrigate normally, your fruit is more vulnerable because you'll have less foliage," he said.

Shepparton orchardist Andrew Prentice said pears and apples were particularly susceptible to sunburn on consecutive hot days.

"If it gets too hot, the pickers knock off at lunchtime . . . this sort of weather is not what we need in the middle of harvest," Mr Prentice said.

Bureau of Meteorology climatologist Blair Trewin said this week's heatwave was "right up there" with similar events in 1908 and 1939.

"In 1939, temperatures north of the Divide were over 40 degrees for anywhere from five to 10 consecutive days," Dr Trewin said.

"It's unlikely that what we see this week will match what we had north of the Divide in 1939."

Dr Trewin said temperatures this week would be "fairly even" across much of the state.

"It's not one of those events where it's going to be a lot hotter in the Mallee," he said.

Victorian Farmers Federation president Simon Ramsay urged farmers to ensure stock had access to water at all times.

He said farmers, many of whom are also CFA volunteers, should prepare their properties for fire as best they could.

Earlier this week, the Country Fire Authority warned of "a fairly major fire event" as the mercury began to soar.

Meanwhile, rain continues to elude farmers across the state, with not one drop of rain recorded in Swan Hill or Stawell this year.

Less than 1mm has fallen in Horsham, Ballarat, Mildura and Melbourne.

"Parts of eastern Victoria have done a little bit better with some rain from thunderstorms, but rainfall is still well below average," Dr Trewin said.