Perth has sweltered through its hottest day in five years and the hottest February day in 23 years, with temperatures in the city peaking at 42.7 degrees Celsius just after 3:00pm.

Key points: A heat trough and easterly winds combined for a sweltering day in Perth

A heat trough and easterly winds combined for a sweltering day in Perth The heat will continue overnight, with temperatures only dropping to 25C

The heat will continue overnight, with temperatures only dropping to 25C It comes as the third cyclone of the season builds off WA's north coast

It is the highest February maximum since 44.5C was recorded on February 26, 1997, and the overall highest maximum since 44.4C was reached on January 5, 2015.

Perth's coastal suburbs also soared above 40C as east-north-easterly winds held back the sea breeze that usually brings a cool relief in the afternoon.

Swanbourne reached 42.1C, Hillarys Boat Harbour peaked at 40.5C while Mandurah fell just short of the 40C mark, hitting 39.7C.

Trough drags heat south

Bureau of Meteorology spokesman Neil Bennett said the heat was generated by a low pressure trough offshore from the west coast dragging in hot air from the north.

"With the recording station being right on the coast in Mandurah, we usually see temperatures that are cooler but even there it got to 39.7C at 2:30 this afternoon," he said.

"It has been exceptionally hot right across the city, but the real hotspot was up in the north-east through the Bullsbrook area — it was 44.6 degrees there."

The heat is set to linger overnight with the city forecast to only drop to 25C, before a slightly cooler but humid change arrives.

"Tomorrow we expect to see the trough moving inland and that's going to allow some cooler, more humid air to come in off the west coast into the Perth area," Mr Bennett said.

"We'll be under the influence of more westerly winds, so we do expect to see the temperatures drop tomorrow, but it may well be that initially, it could be quite oppressive with the humidity being high, initially anyway."

The easterly winds kept the usual sea breeze at bay. ( ABC News: Hugh Sando )

As the trough moves slowly inland, the heat will be pushed further east across the state, elevating the fire danger and bringing the risk of dry lightning strikes.

Eastern parts of the South West Land Division will experience severe to extreme heatwave conditions from Wednesday to Saturday.

Cyclone Damien developing

Meanwhile, the north of the state is bracing for Australia's third tropical cyclone of the season.

A tropical low is predicted to develop into a cyclone, to be named Damien, offshore from the western Kimberley by Thursday and intensify into a category 3 system before making landfall along the Pilbara coast on Sunday.

A cyclone, to be named Damien, is set to cross the coast near Karratha. ( Supplied: Bureau of Meteorology )

"The current track suggests that a crossing somewhere on the Pilbara coast between Karratha and Onslow is likely and at that point we expect it to be a severe tropical cyclone — a category 3," Mr Bennett said.

The tropical low has already delivered falls of nearly 200 millimetres in the past three days, with the heaviest 24-hour total being 162 millimetres at Truscott Island.

A flood watch alert is in place for the North, East and West Kimberley rivers.