Parts of western Sydney baked as temperatures neared 40C on Wednesday, maxing out at 39.8C in Bankstown and 39.5C at Penrith after 1pm, both record highs for October.
Closer to the harbour temperatures topped out at 37C about 2.30pm in Sydney's city centre, slightly below the October record.
Strong winds helped it feel slightly cooler in some parts as the mercury rose, while further south Victorians are being hit by destructive gusts.
Widespread gusts between 90km/h to 110km/h were expected in the state, pushing up to 125km/h along the coast, the Bureau of Meteorology's Angus Hines said.
"We'll see those strong winds impact other parts of Victoria, maybe parts of NSW as well, and then tonight, overnight it's really pulling out into the Tasman Sea and conditions are settling down quite significantly over the country,'' Mr Hines said on Wednesday morning.
A large tree fell on a tram near a hospital at Malvern in Melbourne's southeast, closing a major road in both directions for two blocks.
More than 12,000 Victorian households and businesses were off the grid as of 5.15pm AEDT on Wednesday because of unplanned outages.
Racegoers at the Geelong Cup were urged to steer clear of the marquees as strong winds caused races to be abandoned.
In western Queensland, the mercury passed 40C at Charleville and St George, while further north temperatures ranged from 41C to 43C, hitting 42C at Mount Isa.
The heat continued moving east after monthly records fell at Birdsville (46.1C) in the state's southwest, and Bourke (44.8C) in far north NSW on Tuesday.
"Both of these temperatures were state records for the warmest ever temperature recorded in October anywhere in the state," Mr Hines said.
"These are typically temperatures that we would only see in a summer heatwave, but it's still spring."
Temperatures in NSW were 10-12C higher than average as total fire bans were declared for much of the state on Wednesday..
Taree on the state's mid-north coast hit 41C while Walgett in the northwest also passed 40C.
Dozens of fires were burning across the state on Wednesday afternoon but many of them were under control.
The hot and windy conditions could put animals in harm's way, International Fund for Animal Welfare officer Robert Leach said.
"Animals like koalas are only just recovering after the devastation of Black Summer, we cannot afford another catastrophe."
NSW authorities warned walking tracks and remote campgrounds in national parks across the areas could be closed at short notice.
Cooler temperatures were expected for Sydney on Thursday, but in other areas the mercury is tipped to remain high on Friday and into the weekend.
Maximum temperatures were forecast to reach 35C in Darwin and Brisbane on Thursday, with 22C forecast in Sydney and 18C in Melbourne.
11°C