Residents in bushland areas of Sydney, the Hunter and the Illawarra-Shoalhaven are bracing for "catastrophic" fire danger, with authorities telling them to leave now, before their homes are threatened.
Premier Gladys Berejiklian has declared a week-long state of emergency after a catastrophic fire danger rating was issued for Tuesday in the three heavily populated regions.
NSW Rural Fire Service Commissioner Shane Fitzsimmons urged people living in areas facing the worst threat to leave now.
"My advice is to not be there - leaving early is the safest option," Mr Fitzsimmons told reporters on Monday.
"Catastrophic is off the conventional scale. We are talking about indices that go well beyond the old scale of 100."
Mr Fitzsimmons said fires could spread so quickly in such conditions that people find themselves in severe danger before help arrives.
The commissioner pleaded for people to download the Fires Near Me NSW app.
Exhausted firefighters are battling up to 60 bushfires from the mid-north coast to the Queensland border. Three people have died and at least 150 homes have been destroyed in the past few days.
Doctors and paramedics on the ground have treated more than 100 people hurt during bushfires, including 20 firefighters.
"For heaven's sake, stay away from bushland," Ms Berejiklian said.
"You might think you're okay and a few minutes later you won't be. Please heed all the messages you receive. Tomorrow is not the day to be complacent."
Emergency Services Minister David Elliott said residents faced what "could be the most dangerous bushfire week this nation has ever seen".
The RFS warned that people living outside the catastrophic regions also faced serious danger, with "extreme" and "severe" conditions forecast for the state's north and south.
"There is a risk in all bushland areas, stretching from Bega all the way to Byron, from the Victorian border to the Queensland border," a spokesman said.
"Across all fire grounds now we have more than 970,000 hectares that has been burnt or is burning. That is a massive area almost more than the last three fire seasons combined."
Despite concerns from some - including residents in fire-hit areas - about the link between growing bushfire intensity and climate change, Ms Berejiklian insisted now is an "inappropriate" time to discuss the topic.
She said the focus remains on trying to "keep people alive".
In terrifying tales of survival from the weekend, residents in the Northern Tablelands village of Torrington told how they believed they might die huddled in their community fire station during an "apocalyptic" bushfire.
"It wasn't a bushfire, it was a firestorm," Linda Birch told AAP.
Hundreds of schools will be closed across the state on Tuesday, with Education Minister Sarah Mitchell saying "safety remains the number one priority".
A change in wind direction will likely change the direction of ongoing fires, Grace Legge from the Bureau of Meteorology said.
"We're seeing those stronger winds tomorrow and that hotter air mass which is leading to those higher fire dangers," she told ABC News.
Defence Minister Linda Reynolds said while ADF personnel were not trained firefighters, they could provide other support.
Prime Minister Scott Morrison on Saturday said reservists could potentially work on firebreaks and help with accommodation and catering logistics.
Of the 64 fires burning across NSW, about 40 remain uncontained.
The declaration of a "catastrophic" fire danger rating for Sydney is a first, with high concern for the urban fringe and bushland suburbs.
All state forests from Sydney to the Queensland border, and in the Northern Tablelands, Central West and Moss Vale, are closed until further notice.