A bushfire that was threatening lives and homes in the shires of Donnybrook-Balingup and Nannup has been downgraded to a watch and act after more than 200 volunteer and career firefighters worked throughout the night to contain it.

Key points: Fire crews are hoping to control the blaze today amid favourable conditions

Fire crews are hoping to control the blaze today amid favourable conditions Authorities say they are unaware of any homes being lost, despite earlier reports

Authorities say they are unaware of any homes being lost, despite earlier reports A warning for the townsite of Balingup has been downgraded to an advice

The blaze has burnt through more than 3,300 hectares of land since it started on Tuesday, but authorities said several homes had been saved thanks to the actions of firefighters.

It is understood to have burnt through pine plantations and state forest, but reports two homes had been lost have been dismissed.

Five structures thought to be sheds were confirmed as being destroyed, along with some machinery and vehicles.

The fire remains at watch and act level in parts of Southampton and Cundinup, while the townsites of Balingup, Kirup, Greenbushes and Nannup are now on advice level warnings.

Fire crews are hopeful of bringing the blaze under control soon. ( Supplied )

Easing winds and rising humidity had created more favourable conditions overnight, which continued this morning.

Aerial support helps mop up

DFES incident controller Mark Bowen said the outlook for the fire was "a lot better" today.

Sorry, this video has expired Bushfire burning near Southampton in WA's South West

"The crews that were out there overnight have done a tremendous job," he said.

"The fire is reasonably well contained. Obviously that can change over the day depending on the weather but things are looking a lot more positive than they were yesterday.

"There was a home that received slight damage to an outlying structure and then we've had some damage to other structures in the area like sheds, but nothing that anyone lives in.

"We're hoping to have it fully under control today but obviously there'll be hotspots."

Aerial support has resumed to help fire crews mop up the blaze.

DFES district officer Alan Gale warned against complacency.

Easing winds and rising humidity created better conditions for fire crews overnight. ( Supplied )

"We've got a lot of dry fuel out there still and of course if conditions did change, then we would have to look at changing the warnings again," he said.

"We still ask people to remain vigilant.

"There are still road closures in place and some areas that are unsafe."

Weather conditions 'benign' today

Bureau of Meteorology duty forecaster Noel Puzey said there was very little in the way of weather conditions to worry firefighters today.

"We've got a fairly benign scenario with generally light winds, perhaps with a west- to south-westerly drift throughout the day, some partly cloudy conditions, fairly humid at the moment as well," he said.

"Those things should make it fairly straightforward to fight the fire from a weather perspective [and] might give them a better chance to get it under control today."

The DBCA said the fire started near Radiata Road in Southampton and investigations were underway into whether it was sparked by machinery.

A number of roads are still closed in the area, and authorities are urging motorists to reduce speed and drive carefully due to smoke.

The Bibbulmun Track is also closed between Spring Valley Road and the Southampton Bridge, and the Blackwood Campsite and Wrights Bridge Recreation site are also shut.