Forecaster Michael Efron said northerly winds would draw hot air from inland, likely causing temperatures to spike at 38.9 degrees.

The hottest day so far this year was on January 3 when temperatures reached 37.9 degrees.

Meanwhile, Portland is set for a high of 38 degrees and Hamilton a top of 40 degrees.

The temperatures mean the Warrnambool line V/Line services will operate on an extreme heat timetable on Thursday.

Heat speed restrictions are enforced on Victoria's regional rail network when temperatures exceed 36 degrees to reduce the likelihood of train and track faults and keep passengers safe.

The department of transport also warned motorists to prepare for the hotter conditions with vehicle breakdowns "more likely" in extreme heat.

"It's not unusual for hot weather to result in some bleeding of the road surface, where the road surface becomes soft and sticky," a statement said.

Mr Efron expected temperatures in Warrnambool would drop to a more comfortable maximum of 34 degrees on Friday with increased humidity and thunderstorms a possibility late in the day.

He said there would be increased storm activity on Saturday when temperatures reached a high of 25 degrees.

"Through the late part of Saturday we will see south westerly wind picking up," Mr Efron said.

The temperatures could mean maximums won't reach above 40 degrees this month with one January day last year reaching as high as 44.3 degrees.

Moyne Shire Council warned that people over 65, the very young and those with a pre-existing medical condition were most at risk during high heat.

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