Sooty rain fell down the east coast, from Sydney to Melbourne, with "torrential" rain reported in some parts of New South Wales (NSW).
But on Sunday night officials warned temperatures would rise by Thursday.
They also said huge fires in Victoria and New South Wales could meet to create a larger "mega blaze".
"There is no room for complacency," NSW state Premier Gladys Berejiklian warned on Monday morning.
"This morning it is all about recovery, making sure people who have been displaced have somewhere safe."
Despite the respite, haze pollution remained dangerously high.
The National Gallery of Australia in Canberra was shut on Monday "to mitigate any risk to the public, staff and works of art on display". The Australian National University in the capital is also closed.
Further south, Victoria's Bureau of Meteorology warned that visibility in Melbourne was less than 1km (0.62 miles) in many parts of the city and its surroundings.