NSW Premier Gladys Berejilkian attributed the seven-day measure to the need to save lives and also ahead of dangerous weather conditions that could worsen the blazes, reports Efe news.
At least 100 people have been injured, including 20 firefighters, in the blazes that escalated on Friday, according to the region's ambulance service.
More than 60 fires continue to burn across the state, of which 40 are yet to be contained, the Rural Fire Service said.
"Many of these fires won't be contained ahead of tomorrow's dangerous fire weather. Catastrophic fire danger has been declared for Tuesday in Sydney and Hunter areas," the emergency service tweeted.
Temperatures above 35 degrees Celsius were expected on Tuesday, with dry conditions and strong winds.
In addition to NSW, where fires have devastated 850,000 hectares of land since the beginning of the year, in parts of the state of Queensland, 50 bushfires have also been declared in recent days.
The fire season in Australia varies by area and weather conditions, although they are generally recorded in the southern summer (between the months of December to March).
In recent years, fires in Australia - which this year has also suffered severe drought - have increased in intensity, with many experts linking them to climate change.
The worst fires in the Oceanic country in recent decades occurred in early February 2009 in the state of Victoria. They caused 173 deaths and 414 wounded, and burned an area of 4,500 sq.km.
( With inputs from IANS )