Clouds have gathered over Australia's largest city, Sydney, for the first time in days, bringing high humidity and cooler temperatures.
But forecasters doubt they would produce heavy rain needed to douse scores of bush fires.
Officials say the respite will only be temporary and that high temperatures approaching 38°C and dry winds are expected.
Firefighters have used the lull to build firebreaks and to back burn in an attempt to contain about 100 blazes, many of them burning in tinder-dry forests around Sydney.
New South Wales Rural Fire Service spokesman Mr John Winter warned residents against adopting "a false sense of security".
Large fires are burning in the Hawkesbury region, 40 miles north of Sydney, the Blue Mountains, 50 miles to the west, and Shoalhaven district, 120 miles south.
While no deaths have been reported, thousands of people have taken part in evacuations and about 170 homes have been destroyed.
Around 20,000 firefighters, most of them volunteers, have battled the blazes.
Authorities say nearly half the fires have been set deliberately, many by teenagers and children. Police have arrested 23 suspects, including a nine-year-old boy.
PA