Updated 4.51pm: A cool change has helped firefighters contain a major bush fire near Katoomba in the New South Wales Blue Mountains.

One home and a large area of bush has been destroyed by the fire, which authorities now believe was deliberately lit.

Rob Rogers from the New South Wales Rural Fire Service says fire investigators are now looking for clues of the cause.

He says they are on the scene today, trying to determine where and how the fire started.

Mayor Mark Greenhill is praising the community's resilience and praying for one family who have already lost their home.

He says it brings back memories of last year, and the damage and devastation is extensive.

Ben Shepard from the Rural Fire Service says it does remain dry out there.

"It has the potential every now and then to flare so residents in that area are going to have to remain vigilant and just listen to the firefighters on the ground, we've got a lot of work to do before we can get this fire under control."

Around 50 firefighters are using water-bombing helicopters to contain the fire's spread.

Meanwhile total fire bans are in place in parts of Western Australia's Gascoyne, Central Wheatbelt and Great Southern regions, following a catastrophic fire danger rating.

Twenty-three shires are subject to the ban, which prohibits people from doing anything that might start a fire including lighting open fires and carrying out work such as grinding or welding.