Chief health officer Dr Rosemary Lester has advised local authorities and service providers that elderly, ill and isolated residents are most at risk.
Warrnambool's temperature is tipped to hit 31 on Thursday and 32 Friday, with an overnight minimum of 23. Hamilton can expect 36 and 35, with an overnight low of 24. Colac will hit 35 and 37, with 22 overnight, Casterton will swelter in 38 and 35, with 25 overnight, Port Fairy can expect 31 both days, dropping to 22 overnight, while Portland will be cooler with 28 and 27 during the day and 20 overnight.
Fire authorities are warning of severe fire conditions, especially on Friday .
Regional CFA operations officer Steve Giddens said grass was fully cured and fires would be hard to stop.
"People need to be prepared and even if they are camping along the coast should have escape plans ready," Mr Giddens said.
"In hot conditions people need to hydrate and check on their neighbours."
Glenelg Shire Council has urged people to keep cool, drink plenty of water, stay out of direct sun and look out for others.
Warrnambool City Council said yesterday it did not expect weather to be as extreme in Warrnambool as other parts of the south-west, but would monitor the situation.
The Department of Health has official heat thresholds for each region, calculated using the forecast daily maximum and the forecast overnight minimum. It has determined the south-west's threshold at 30 degrees.
This week's forecasts are similar to the start of 2012 when most parts of the region sweltered on January 2 in above-40-degree heat. The rest of the year turned out to be slightly drier than 2011.
Warrnambool's total rainfall was 710.4 millimetres, with the wettest day of 29.8mm on May 25. Its highest temperature was 42.7 degrees on January 2 and the coolest minus 0.8 degrees on June 27.
Port Fairy received 764.6mm of rain, including 35mm on February 11. Its hottest day was 42.6 on January 2 and the lowe st 1.6 on June 28.
Mortlake had 547.6mm, including 19.6mm on May 2. Its hottest day was 42.7 on January 2 and lowest minus 1.5 on August 13.
Hamilton had 608mm rainfall, including 34mm on May 25. Its hottest day was 42 degrees on January 2 and coolest was minus 0.4 on July 8.
Portland had 637.8mm, including 32.4mm on June 22. The hottest day was 40.2 on January 2 and the coolest 3.1 on June 28.Colac had 516.2mm, including 24.4mm on December 1.
Its hottest day was 40.8 on January 2 and the coolest was 1.4 on September 29.