Much of south-east Australia will roast over the weekend, lifting fire risks to "catastrophic" in parts of South Australia, and sending the mercury in western Sydney to levels more akin to the outback.
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"We're likely to see large parts of NSW move into [a] severe heatwave over the weekend," Andrea Peace, a meteorologist with the Bureau of Meteorology, said.
Sydney's top is expected to reach 31 degrees on Saturday and 38 on Sunday, but the most intense heat will be felt in western suburbs. Penrith is likely to reach 42 and 45 degrees on those two days.
"Parts of Sydney are going to be hotter than Bourke [in the state's far north-west]," Kim Westcott, a meteorologist with Weatherzone, said. "You don't tend to see it that often."
The NSW Rural Fire Service lifted its fire danger rating to "very high" on Saturday for a region arcing from the southern Riverina, taking in Sydney and the Hunter, up to the north-west.
"It is likely we'll see total fire bans for Sydney and the Hunter on Sunday," Ben Shepherd, senior RFS spokesman, said. "We are fortunate we aren't seeing the conditions they have south of the border [in Victoria] and in South Australia."
South Australia's "catastrophic" fire threat on Saturday applies to its Lower South East district and the Mount Lofty Ranges, the latter for the first time since a record Australia-wide burst of heat in January 2013.
Victoria will have a total fire ban for Saturday with Melbourne expecting a top of 42 degrees.
While southern regions can expect a gusty cool change - Melbourne may drop 20 degrees in a couple of hours on Saturday evening - Sydney is likely to only see a weak cooling of late on Sunday before temperatures rise again on Monday.
The greater cloud cover will lift humidity in Sydney on Monday so that some people "might struggle more" than during the weekend, Ms Westcott said.
Sydney's forecast for Monday is 33 degrees and 41 degrees in Penrith.
The heat will subside from Wednesday but is likely to rebound by Friday.
A heatwave the following week could be as hot and even longer than this weekend's, Ms Westcott said.
Despite the extent of this weekend's heat and its intensity, the Australian Energy Market Operator does not expect any outages.
"The supply/demand balance looks good across all regions," an AEMO spokesman said.
Likewise, the Reliability and Emergency Reserve Trader process set up to create a strategic reserve of 1150 megawatts of electricity is not expected to be triggered, despite some media reports.
"We do not anticipate the need for RERT to be enacted this weekend," the spokesman said.
Weatherzone is owned by Fairfax Media, publisher of this website.
