A "classic heatwave set-up" will deliver the hottest run of temperatures for south-eastern Australia since just before Victoria's Black Saturday bushfires in 2009, the Bureau of Meteorology said.
While forest conditions are not as dry as the period before those bushfires, grasslands are parched and may pose some high fire risks, senior bureau forecaster Terry Ryan said.
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A week of heat ahead
Northerly winds are set to bring intense hot weather to Melbourne with tempertures exceeding 40 degrees around Victoria this week.
The bureau raised Melbourne’s forecast maximums slightly for some days this week, just as the Australian Open tennis grand slam tournament gets under way.
The city can expect a 35-degree maximum on Monday, with days of 41, 39, 41 and 40 to follow.
The city averages 1.5 days a year of 40 degrees, so to have three in a single week is very unusual, Mr Ryan said.
The increase appears to be one signal of climate change, nudging temperatures higher, he said.
Since about 1998-99, "there's been a lot more hot days", Mr Ryan said, adding that the extra degree or so was turning 39s into 40s.
For the tennis, the tournament will struggle to exceed the extreme heat of the 2009 Australian Open. The first week had three days of 35 degrees or more, including 40.5, and then three days of 43.4, 44.3 and 45.1 degrees in the second week, said Weatherzone's senior meteorologist Brett Dutschke.
There's an advantage being a high-ranked player during hot spells at the tennis as they tend to play on the main courts under a roof, and also play at night when temperatures ease off, he said.
Conditions in the second week should be milder for those who make it through, with the mercury climbing to the mid-30s on perhaps one day. "The second week is looking noticeably cooler," he said.
There may be only limited relief in the coming days with sea-breezes arriving from the west, potentially easing Tuesday’s heat – but only temporarily as the heat reforms, he said.
"The heat is never far away," Mr Ryan said.
Sydney will be spared much of the extreme heat over the coming week, with more comfortable temperature ranges of 25-30 degrees, thanks to daily sea breezes, the bureau said.
Areas to the west, such as Penrith, though, will have maximums climbing to 37-39 degrees from Wednesday to Saturday.
The NSW Riverina town of Wagga Wagga, which reached 39 degrees on Sunday, can expect the mercury to hit 38-43 degrees from Monday to Saturday.
Inland Victoria will also have very hot conditions in coming days.
Shepparton, for instance, will endure five days of 40-plus maximums from Monday, with 44 forecast for Thursday and Friday - more than 14 degrees above the long-term January average for the town.
Overnight temperatures will not offer much relief.
Minimums in Melbourne will not dip below 22 from Tuesday until Saturday, while Friday's low is unlikely to drop below 28 degrees on current forecasts.
Sydney's minimums will be a muggy 19-21 through most of the week.
One reason the heat may linger is that two tropic lows on either side of Australia to the north may form into tropical cyclones.
The potential cyclone to the north-west, which may bring heavy rain to the Top End and the Kimberley, may also bring rain to south-eastern Australia and help break up the heat by about Tuesday January 21, Mr Ryan said.
Tuesday and Friday are looking likely to be the most dangerous, with temperatures across northern Victoria and into southern NSW well into the 40s.
On Friday, those areas may get winds averaging 35-40km/h with gusts to 60km/h, Dr Dutschke said.
"It will probably be a pretty rough day," Dr Dutschke said.
Adelaide and the rest of South Australia will also have very high temperatures.
The South Australian capital can expect five 40- to 43-degree days from Monday, the bureau said.
In the west, meanwhile, about 150 firefighters are still battling a blaze in the Perth hills, which destroyed at least 27 homes, AAP reported. The city had its hottest day in more than six years - and its warmest ever night - over the weekend, with 43.3 degrees and 41 on Saturday and Sunday, respectively.
For some regions of Australia, it will be the second belting of heat this year, after an earlier heatwave broke records at 34 weather stations, including the most at any site with more than 40 years of data – at Narrabri in northern NSW.
While Melbourne is likely to dodge a return of the fierce heat next week, inland areas should warm up again after the weekend, Dr Dutschke said.
"It should just be getting into the mid- to high-30s into next week, at least in northern Victoria," he said.
"But there's still plenty of time, possibly by the end of the month, to have intense heat returning."
The hot spells come after 2013 was declared by the bureau as Australia’s hottest year in more than a century of standardised records, with the background warming from climate change a contributing factor.
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