A smoke alert has been issued for Perth's southern suburbs as a suspicious bushfire continues to burn more than 100 kilometres away in Western Australia's South West.

An emergency warning for the fire has been downgraded after firefighters managed to contain the blaze overnight, about 15 kilometres north of Leschenault on the outskirts of Bunbury.

People had been urged to evacuate the area in the Shire of Harvey late on Monday afternoon as authorities warned lives and homes were at risk.

Emergency services said while the fire threat had eased, smoke from the blaze was now affecting Perth's southern suburbs.

If the smoke is particularly thick, residents should shut their doors and windows and turn off air conditioners.

The fire is believed to have been deliberately lit. ( Supplied: Sara Ward )

Anybody with asthma or pre-existing respiratory or cardiovascular illnesses is urged to follow their treatment plan.

Motorists have also been told to take extra care.

The smoke is expected to clear over the southern suburbs and surrounding areas by early afternoon.

The fire, which is believed to have been deliberately lit, has burnt through 200 hectares after starting near the intersection of Treasure Road and Wellesley Road North.

The blaze is stationary, contained and under control.