A southerly change with powerful gusts up to 80mph (128km/h) threatened to spread huge fires raging in New South Wales (NSW), officials said.
In the neighbouring state of Victoria, army helicopters have been deployed to evacuate people trapped by the flames.
Since September, fires in Australia have killed at least 23 people.
More than 1,200 homes have been destroyed and millions of hectares of land scorched. Although much attention has centred on worst-hit NSW, every state and territory has been affected.