WA Bureau of Meteorology duty forecaster Jim Richardson said on Tuesday a prolonged high pressure system in the Bight was still pushing hot air down from the Kimberley, Pilbara and Gascoyne to the west coast, delaying the usual early afternoon seabreeze and prolonging the hot weather and humidity throughout the day.

With three days of 40C forecast between Tuesday and Sunday, the monthly metro record for 40C days in December would also likely fall, with one already recorded in Mt Lawley, while the three recorded at Perth Airport had already set a new benchmark.

The most 40C days in any month is seven, for January in 1956 at the airport, while four days above 40 have been recorded on several occasions in January and February for the metro area.

The highest number of 40C days in a year is nine, set in 2007 (three in January, one in February, three in March and two in December). That record could also be broken by the time Santa has come and gone for the year.

It is not only December that has felt extreme temperatures, with the mercury on November 16 hitting 40.4C, a new record high temperature for that month.