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

Almost 80 fires are burning from near Nowra all the way up to the Queensland border with bureau state manager Ann Farrell saying the situation is changing "very rapidly".

"We're looking at a wind change that's really going to worsen things dramatically through the night," Ms Farrell said in Sydney.

A low-pressure system moving across the state is bringing "strengthening" winds that will increase fire danger on Tuesday evening.

The dangerous southerly change was expected to move through Sydney about 7pm and progressively track up the coast.

"It won't be until tomorrow morning that we see it clear the northeast of the state," Ms Farrell said, adding 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."

NSW Rural Fire Service commissioner Shane Fitzsimmons said firefighters and affected communities were facing "a really long night".

He said there would sadly be no rainfall in the southerly change and warm, dry conditions are 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."

Australian Associated Press