Humans and animals alike have been warned to take caution as the risk of heat-related harm rises with record-breaking temperatures.

A spring heatwave has cranked up the mercury, with Queensland and NSW both hitting their highest recorded temperatures for October.

Birdsville in Queensland's southwest reached 46.1C while Bourke in far north NSW reached 44.8C on Tuesday.

Parts of Sydney were expected to flirt with maximums of 40C on Wednesday, according to weather bureau senior meteorologist Angus Hines.

The temperature had climbed to nearly 37C in the city centre by early afternoon, while western and southwestern suburbs had readings above 38C.

Walgett, in northwest NSW, had a temperature of 39.1C.

"These are typically temperatures that we would only see in a summer heatwave, but it's

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