Firefighters have a long night ahead of them with strong winds expected to reach northern NSW fire grounds around midnight.

The Bureau of Meteorology on Tuesday afternoon warned things will get worse before getting better.

More than 70 fires are raging across NSW with 30 out of control and 10 subject to emergency warnings.

The Greater Sydney, Greater Hunter and Illawarra-Shoalhaven regions have been issued catastrophic danger ratings.

BOM state manager Ann Farrell says a low-pressure system moving across the state is bringing “strengthening, hot, dry winds” that will increase fire danger on Tuesday evening.

Ms Farrell says a dangerous wind change is expected to hit Wollongong about 5pm, southern parts of Sydney about 6pm and Port Macquarie by midnight.

The change will bring vigorous southerly winds which could rapidly expand the length of fire fronts.

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“Winds that are currently moving from the northwest are going to change to southerly and track up the coast,” she said in Sydney.

“And when that change happens … the fires will be fanned in a different direction. They’ll be very gusty and quite unpredictable at times. So that will make fire conditions much more dangerous.”

Ms Farrell said the wind change could see temperatures drop about 10 degrees.

“Things are going to be pretty difficult for some time tonight before we see any real improvement in fire fighting conditions,” she said.

NSW Rural Fire Service commissioner Shane Fitzsimmons says Tuesday is playing out as predicted with average winds up to 60km/h across the ranges with gusts over 70km/h.

He said firefighters and affected communities were facing “a really long night”.

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“This southerly is going to continue through and those strong winds are not going to get to northern areas of NSW until midnight or the early hours of this morning across some of these fire grounds,” he said.

Mr Fitzsimmons said there would be no reprieve for firefighters with no rainfall in the southerly change and warm, dry conditions set to dominate in the days and weeks ahead.

“As we look out a little bit further, we’re expecting a cyclical pattern of increasing temperatures as we head into the end of this week and indeed as we head into next week,” he said.

Premier Gladys Berejiklian in parliament on Tuesday said the state was expecting “the worst fire season imaginable”.

“It’s barely the end of spring and we’ve already witnessed that already,” she said.

“Extreme weather conditions due to the deep drought are likely to continue over the next few months and beyond.”

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AAP