A multi-agency investigation has been launched after waterbombing aircraft were grounded due to drones flying near bushfires in Western Australia's south-east.

More than 300 firefighters and support personnel have been battling several fires in the Shire of Esperance.

Six water bombers and two support aircraft were called off for two to three hours on Saturday, when a pilot spotted a drone in Howick, the site of the region's worst blaze.

A second drone was later seen in the same bushfire warning area.

Multiple fires have threatened lives and properties across the Shire of Esperance. ( Supplied: DFES incident photographer Evan Collis )

Department of Fire and Emergency Services (DFES) incident controller Danny Mosconi said it caused a significant setback for firefighters.

"We estimate the suspension of the aircraft prevented nearly 100,000 litres of water and retardant [from] reaching the fire ground," he said.

"We had to stop them [aircraft] until we could confirm the drones were no longer in the sky because they pose a significant risk to the firefighters, pilots and crews."

The drone operators are yet to be found and an investigation involving DFES, WA police, the Department of Biodiversity, Conservation and Attractions, and the Civil Aviation Safety Authority (CASA) is underway.

Texts were sent to nearby residents, asking the drone operator to stop immediately. ( Supplied: DFES )

No property damaged, campers evacuated

Firefighters are working to control several blazes in the Esperance region, some of which have been burning for up to six weeks.

There have been no reports of property loss but the fires have burnt through more than 300,000 hectares in areas including Beaumont, Howick, Boyatup, Buraminya, Cape Arid National Park and Cape Le Grand National Park.

Emergency services were also concerned about tourists, with more than 2,000 people expected to visit the region during the Labor Day long weekend.

Mr Mosconi said campers were allowed into Lucky Bay but other campsites including Alexander Bay had been evacuated.

"We're asking people in the area to remain vigilant," he said.

"Esperance itself isn't affected by the fires but if people are heading into those affected areas, stay up to date by visiting the Emergency WA website."

Conditions are expected to ease over the next few days.

The worst of the fire alerts was downgraded to a Watch and Act level on Saturday. ( Supplied: Parks and Wildlife Chris Taylor )

Why are drones dangerous to aircraft?

CASA spokesman Peter Gibson said those who flew drones near fires put lives at risk.

"A little drone could cause catastrophic situations with small aircraft, particularly with helicopters," he said.

"You could bring a helicopter down if a drone hit the rotor blades, particularly the tail rotor blades."

He reminded drone operators it was against the law to fly a drone in the vicinity of an emergency situation.

"If you see a bushfire, keep your drone on the ground at all times," he said.

"Each situation is different, it could be 1 kilometre from the fire or 20 kilometres from the fire [that is considered too close], depending on the size of the fire."

CASA has already issued two fines this year to drone operators who flew near fires in Australia.

Last year, about 60 fines were issued for drone offences, including a few fire-related fines.

"The fines can be more than $10,000 and if you put lives at risk with your drone, you could end up before the courts and that could result in a jail term of up to five years," Mr Gibson said.

DFES has urged drone operators to stay away from emergency situations and to download the CASA app, which provides information on where drones can be used.