A severe heatwave will cross the Border region during the first half of the week and the temperature should exceed 40 degrees for the first time this summer, the Bureau of Meteorology has warned.
The Bureau has predicted "severe" heatwave conditions with maximum temperatures of 40 degrees on Tuesday and Wednesday.
Conditions should ease to a "low-intensity" heatwave by Thursday with a high of 34 degrees.
It comes after a top temperature of 36.4 degrees was recorded in the month of December, and 41.8 degrees in November.
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A fire danger rating of very high is in place in the Riverina across the week until Thursday. The Country Fire Authority has also classed the North East's fire risk as very high on Wednesday and Thursday.
NSW Meteorologist Melody Sturm said a high pressure system over the Tasman Sea was pushing warmer air south.
Victorian senior forecaster Chris Arvier said the North East would be unaffected by a cool change forecast to make temperatures to drop across most of the state following Monday's heat.
"That cooler air won't be pushing much north of the ranges, so places across the north along the Murray aren't expected to see temperature drop much in the overnight period," he said.
Mr Arvier warned fire danger and heat health warnings were in place.
"When we see the minimum temperature remain quite high overnight it does mean people vulnerable to heat stress, the elderly, young, might have increased exposure to those risks," he said.
Australian Red Cross' Janie McCullagh has urged residents to be prepared and learn the signs of heat stroke.
"Extreme hot weather can cause serious health problems. More Australians have died as a result of heatwaves than floods, bushfires or cyclones," she said.