PERTH'S heatwave has sparked several fires and the mercury is expected to hover around 40C until the New Year.

Children playing with fire crackers are believed to have started a fire that threatened homes in Kenwick, Perth's southeast, yesterday about 9.30pm.

A Department of Fire and Emergency Services spokesman said the blaze started in bush and burnt about 2ha.

In a separate incident, exhaust from a car is believed to have started a fire in Upper Swan, in Perth's northeast, when a man doing fencing work parked his car on dry grass.

About $500,000 worth of damage was caused when a faulty air conditioner started a fire about 12.30am this morning in Embleton, in Perth's north-east.

One person was believed to have been in the house but they escaped injury.

In Chidlow, a car suspected of being involved in a burnout caught fire at 12.30am, costing $8000 worth of damage.

Two youths have also been questioned by police over two incidents in which fires were believed to have been deliberately lit.

The first was in Darlington, in Perth's east, at 7pm yesterday where witnesses reported hearing an explosive banging sound before a fire was spotted in some scrub.

The second was in South Lake, in Perth's south, about 10pm where tree scrubs and grass were set alight.

The DFES warns that residents need to be careful and aware of potential fire hazards, especially for the remainder of the week as Perth swelters in the heat and more accidental fires are likely.

The Bureau of Meteorology forecasts the chance of a late storm today, but the temperature is still expected to reach 41C.

There will be isolated showers and thunderstorms tomorrow, but again the temperature will be 40C.

Saturday will be a very hot 42C and Sunday will be 41C.

December is expected to finish with 38C on Monday, before a slightly cooler 34C on New Year's Day.

Surf Life Saving WA has also been busy as many flock to the beach to beat the heat.

Shopping centres were also swamped on Boxing Day and were expected to attract more visitors for the remainder of the week as people attempted to escape the heat with some air conditioning.