Bushfire warnings have been issued for parts of each state, with particular emphasis placed on looking after the elderly and people with medical conditions.
Play at the Australian Open in Melbourne may be affected if temperatures surpass 40C, with referees having the discretion to stop matches, in line with the tournament's extreme weather policy.
Cyclists' competing at the Tour Down Under in South Australia have also been placed on high alert.
NSW: 40C+
NSW Rural Fire Service spokesman Greg Allan said people should make their own bushfire survival plan as each household has different needs.
On Saturday the mercury is set to top 45C at Hay and Ivanhoe in western NSW and 43C in Penrith in western Sydney.
The NSW RFS has issued a severe fire danger warning for Sydney, the central ranges, northwestern NSW, the Illawarra, the Hunter and the southern and northern slopes on Thursday.
Victoria: 40C+ hottest in three years
Temperatures in Melbourne are expected to climb to 39C on Friday, the Bureau of Meteorology says, with regional towns expected to crack 40C.
Total fire bans have been declared for Thursday in several parts of the state including the Mallee, Wimmera, South West, Central and North Central areas and health warnings have been issued.
South Australia: 27C+ at night, 40C+ days
Temperatures hit 41C in Adelaide early Thursday afternoon and are unlikely to dip below 27C overnight as the state braces for another scorcher on Friday.
The needs of the elderly, frail, young and those with cardiac and kidney complications could stretch medical services during South Australia's three-day heatwave, SA Health says.
Queensland, Tasmania and WA
Queensland, Tasmania and Western Australia look set for much cooler temperatures over the weekend.
Perth is expecting it's hottest day on Saturday as the mercury looks set to reach a high of 33C.
Queensland's southern regions look set for temperatures in the early 30s, with central areas expecting seasonal norms nudging towards the late 30s.
Hobart is set for a high of 33C on Friday, before showers push temperatures to 20C on Saturday.
