Heatwave conditions and strong winds have fanned flames in Australia's Blue Mountains in New South Wales, with at least one home destroyed on Sunday.

Video shared by the New South Wales Rural Fire Service showed the Gospers Mountain bushfire reaching the top of trees and moving from treetop to treetop in Wollemi National Park.

Firefighters said flames were reaching 60-70m.

Bushfires have killed at least four people across the east coast of Australia since November, burnt about a million hectares of farmland and bush, and destroyed more than 400 homes.

The Gospers Mountain fire has burnt through more than 375,123 hectares of brush alone, according to the Rural Fire Service.

Firefighters are battling more than 100 blazes across New South Wales, with conditions expected to worsen in coming days.