Temperatures are soaring into the 40s across large swathes of Australia's southeast, providing fire authorities with their first major test this summer.
Total fire bans are in place on Monday across the three NSW Riverina districts and in six Victorian districts including the Mallee, Wimmera, Northern Country, North East and North Central and East Gippsland.
Two homes are also feared to be either damaged or destroyed by an uncontrolled bushfire in the hills southeast of Adelaide in South Australia.
While temperatures are expected to settle after a wind change in Victoria on Monday, the heat will linger in Sydney across NSW. Southern NSW will enjoy relief on Tuesday, while Sydney will cool down on Tuesday night.
In Tasmania, cooler conditions are expected from Monday afternoon.
Bureau of Meteorology NSW manager Agata Imielska said Australia Day temperatures in western Sydney would top 40C but coastal areas would benefit from a sea breeze, reaching about 35C.
Overnight temperatures on Monday would remain high, approaching 30C.
But heat records are unlikely to tumble this week.
"Tomorrow we will see a change move across (NSW), starting across the south, bringing cloud and rain in the morning to southern areas, so we will see cooler temperatures," Ms Imielska told reporters on Monday.
Canberra will cop weather in the low-30s on Australia Day before a late shower.
NSW Surf Life Saving chief executive Steve Pearce said strong northerly gusts and strong ocean swells - combined with high temperatures - would make conditions challenging for swimmers and lifeguards.
Five people have drowned in NSW in the past seven days while another two men drowned in separate incidents in Victoria on Saturday.
Drownings are 2.4 times more likely to occur in Australia on public holidays and young men are almost twice as likely to drown on a public holiday.
"We see it to be the largest Australia Day in the past five years," Mr Pearce said.
"It's going to be an absolutely splendid day for the beach but it's vitally important that when you come down to the beaches, you must look for those patrolled locations, where the red and yellow flags are flying."
NSW Ambulance's Kay Armstrong said people needed to stay hydrated and look out for each other during the heatwave.
Australian Associated Press