Perth is set to equal an 82-year-old record if temperatures top 40 degrees on Monday, Tuesday and Wednesday.
Monday's forecast remains at 42 degrees for the metropolitan area, a figure predicted to be matched on Tuesday, with Wednesday set to be 41 degrees.
Perth reached 40.4 degrees on Sunday afternoon.
"It would put us equal with the current Perth record of four consecutive 40-degree days in 1933," Weatherzone's Brett Dutschke said.
"But back then the four-day average was 42.8...this time we'll average about 41."
A record five straight days of 40-plus is not out of the question, with 39 degrees forecast for Thursday.
"It's a possibility...but it's not likely to get to 40," Mr Dutschke said.
Adelaide holds national capital city record of six consecutive 40-degree days in 1908.
A cooler weekend is forecast for Perth, with Friday 36 and Saturday and Sunday both 31.
