Temperatures have hit 40C across large parts of southeast Australia, as a cool change begins to blow through Victoria.
A cool change has ushered in relief for Melburnians after an extended heatwave but the warm weather will linger in Sydney and across NSW.
Total fire bans have been in place on Monday in 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 a bushfire in the hills southeast of Adelaide in South Australia. The Cherry Gardens blaze in the Mount Lofty Ranges was downgraded to advice level by early Monday afternoon but authorities warn the fire will stay active for days.
But temperatures across Melbourne and Victoria's southwest and centre - which spiked at 42C at Laverton - began on Monday afternoon to settle down, with a wind change bringer cooler conditions.
The same wind change will hit Victoria's east and north later on Monday, easing conditions in places such as Shepparton, which has reached 42C.
The Bureau of Meteorology's Dean Narramore told reporters temperatures in Melbourne dropped 10 degrees in 10 minutes on Monday, and the weather would remain cool on Australia Day.
"It could be a wet morning for Australia Day through Melbourne, but the rain will tend to slow in the afternoon and hopefully a few sunny breaks by the time we get to the mid-to-late afternoon," Mr Narramore said.
"It's only late January and we still have February to go through yet and sometimes we don't get big bursts of heat until early March ... I'm sure we'll see another burst of heat like this in the coming weeks."
In Tasmania, cooler conditions are also expected from Monday afternoon.
Yet while temperatures dipped in Victoria, heat will linger in Sydney and NSW until southern NSW is hit by the wind change on Tuesday.
BOM 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 will remain high, pushing 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.
A boy was also pulled from the water unconscious on Monday in Sackville, to Sydney's northwest, and taken to hospital in a critical condition.
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 until the heatwave subsided.