Victoria is set for another heatwave, with Tuesday declared a total fire ban day across the state by the CFA.
Parts of Victoria will stay above 40C for six days, while Melbourne will be in the 30s for much of the week.
In South Australia, 11 schools have closed as a fire ban was declared state wide. Temperatures are expected to reach 41C in Adelaide.
The Country Fire Service (CFS) says the weather conditions could not be worse, as South Australia faces its second heatwave this month.
"We are facing a horror day when we already have existing fires burning in the state," state coordinator Brenton Eden said.
The heatwave comes as figures from the Victorian Institute of Forensic Medicine show there were 139 more deaths than expected between January 13, when the last heatwave began, and January 23.
Victoria experienced its hottest four-day period for both maximum and average heat, with four days in a row of 40C temperatures felt in Melbourne.
Temperatures were on par with the week leading up to the Black Saturday bushfires of 2009.
In Mildura, there will be seven days in a row above 40C, meteorologist James Taylor said.
"The heat hangs right in there for Mildura with a forecast of seven days straight of above 40 degrees," Mr Taylor said.
"It's due to a pool of hot air over central Australia sweeping across the north of the state this week."
In South Australia, firefighting operations continue at Bangor in the Southern Flinders Ranges and at Billiatt in the Riverland.
WATCH: SBS reporter Karen Ashford has the latest on the SA heatwave
As weather conditions are expected to worsen over the coming days, the CFS said the fire behaviour on the Bangor and Billiatt firegrounds will intensify, posing additional challenges for firefighters.
Attorney-General Robert Clark says everyone can do their bit to check on the elderly and vulnerable during periods of extreme heat.
"Heat is always a difficult challenge for people who are old and vulnerable," Mr Clark said.
"If you've got an elderly or frail neighbour drop in and make sure they're doing okay on a hot day."
