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.

"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."