Residents are urged to avoid or leave bushfire-prone areas.
"It's not another summer's day. It's not another bad fire weather day. This is as bad as it gets," Rural Fire Services Commissioner Shane Fitzsimmons said Saturday during a news conference.
"It is simply not a safe environment, which is why we're making it very clear to people that the only safe place to be is not in at-risk areas," he said.
There are 49 fires now burning across New South Wales, including 17 that are not contained, Fitzsimmons said. About 300 firefighters are actively fighting blazes, and thousands more are on standby, he said.
"We need to be clear that in catastrophic conditions, it is the most dangerous of conditions. Fires will start early, and they will spread very quickly," Fitzsimmons said.
He warned that the fire service does not have the resources to knock on every door to warn residents if fires spread quickly, but he said officials would do their best to save lives and homes.
The "catastrophic" fire danger conditions are forecast for the Greater Hunter, Central Ranges and North Western areas, the Australian Bureau of Meteorology said. Half a dozen other areas face "extreme" and "severe" fire danger warnings for Sunday.
A "catastrophic" warning carries the risk of significant loss of life and the destruction of many homes, according to the NSW Rural Fire Service.
'Unprecedented' weather events
The Bureau of Meteorology warned in a news release Saturday that New South Wales "could experience its hottest February day on record tomorrow, as extreme heat wave conditions peak across the state."
The temperature in one Sydney suburb, Penrith, was forecast to reach 45 degrees Celsius (113 degrees Fahrenheit) on Saturday, it said.
A total fire ban is in place across the whole state for the weekend, as authorities seek to prevent new bushfires igniting. Those could prove extremely difficult to control, given the conditions.
Part of the problem is an approaching weather front to the south that is dragging a buildup of hot air from the interior of the continent down across New South Wales, the Bureau of Meteorology said.
It comes on the heels of a sustained heat wave. January was the hottest month on record for Sydney since 1859, the bureau said, and some places in the state are breaking records for the number of consecutive days at high temperatures.
At the same time, record rainfall in New South Wales last winter led to high plant growth, providing ample fuel for bushfires to spread.
The conditions may spark fears of a repeat of "Black Saturday" in 2009, when soaring temperatures and high winds fanned the flames of a series of bushfires across the state of Victoria. Those fires left 173 people dead, injured 500 more and destroyed thousands of homes.
Some relief from the extreme heat in New South Wales is expected Monday, the Bureau of Meteorology said.