Perth teens charged over Champion Lakes emergency bushfire after ...

The teenagers accused of allegedly starting a bushfire that closed a major highway in Perth's south-east had lost control of a campfire after spraying it with an aerosol can, a court heard.

Two 14-year-old boys allegedly set fire to a property near Mustang Road in Champion Lakes on Monday afternoon, which tore through more than 150 hectares of vegetation.

The blaze triggered the first emergency warning for the Perth metro area this bushfire season, causing authorities to close Tonkin Highway and urge residents to evacuate the area.

It was downgraded to a watch and act later that night.

The blaze caused traffic chaos after it was deliberately lit. (ABC News)

Police said there were unconfirmed reports the fire damaged a number of farming sheds and killed livestock.

The pair were charged with one count each of 'wilfully light bushfire'.

On Tuesday, the Perth Children's Court heard the boys lived together on a nearby semi-rural property.

The court heard the boys wanted to light a "little campfire" at the back.

The state prosecutor told the court the fire was under control until the boys sprayed an aerosol can onto the flames.

The blaze got out of control, and the court heard the pair tried to extinguish the fire by putting sand on top of it but failed to do so and called emergency services.

The court was told the pair handed themselves into Armadale police station and were "showing remorse for their actions".

One of the 14-year-olds pleaded guilty to his charge and was released on bail on the condition he does not possess any lighter or implement which could be used to start a fire.

His lawyer told the court he did not intend to start a bushfire.

"It's not the circumstance of lighting something to watch it go up," she said.

Magistrate Sarah Dewsbury acknowledged the circumstances were "unusual" given the teenagers called police, showed them the site of the initial fire and handed themselves in.

"It seems to be that it's a somewhat different set of circumstances in this case," she told the court.

The other teenager did not enter a plea but was granted bail on the same conditions as his co-accused.

The pair are both due before court on December 17.

Firefighters are also battling a separate blaze that has shut a major road north of Lancelin.

That fire is burning inland of the Wedge Island shack settlement and has closed Indian Ocean Drive between Lancelin and Cervantes.

While it is a remote area, an evacuation centre has been set up in Jurien Bay.

The Lancelin fire is not connected to the Champion Lakes fire.