No deaths had been reported, although officials in Tasmania were still trying to find around 100 residents missing since a blaze tore through the small town of Dunalley, east of the state capital of Hobart, last week, destroying around 90 homes.
No bodies were found during preliminary checks of the ruined houses.
"We are shaping up for one of the worst fire danger days on record," New South Wales rural fire service commissioner Shane Fitzsimmons said.
"You don't get conditions worse than this. We are at the catastrophic level and leaving early is your safest option."
Catastrophic threat level is the most severe rating applicable.
Wildfires have razed 50,000 acres of forests and farmland across southern Tasmania since Friday. In New South Wales, the country's most populous state, the fires had burned through more than 64,000 of land.
Strong winds were hampering efforts to bring the fires under control. Wind gusts more than 62mph (100kph) were recorded in some parts of the state. All state forests and national parks were closed as a precaution.