Virtually all of eastern NSW will face significant risks in coming days ahead of what authorities have declared "an extremely dangerous period of fire danger".

"From Bega to Byron, basically it's the whole length of the state," said Inspector Ben Shepherd, a senior Rural Fire Service spokesman. "This is just a massive event."

While the entire state has a total fire ban declared for Monday and Tuesday, it is Tuesday that is the major concern for authorities.

"The advice for people near Sydney's more forested areas - such as Hornsby, the Blue Mountains, parts of Sutherland Shire and even Lane Cove - is to stay out of the bush," Inspector Shepherd said.

"It's going to be hot, windy and awful, and we don't want you to go into bushfire-prone land at that moment."

A "catastrophic" fire danger is forecast for the Greater Sydney and Greater Hunter areas on Tuesday, with Sydney notching its first such levels since ratings were revised after Victoria's 2009 Black Saturday fires.

The RFS says people in catastrophic fire danger areas should stay away from bushland areas and seek safety in a large town, city or shopping centres.

"If you are unable to leave, identify a safe location which may be nearby," the statement said.

The RFS also said schools in high-risk areas will be closed, urging people to start making arrangements now.

NSW TrainLink is working closely with the Rural Fire Service and road and rail infrastructure managers to determine when it is safe to resume services.

Among the effects of the fire bans will be the closure of some state parks.

National parks north of the Hawkesbury River, for instance, are set to be closed from Monday at 8am until further notice.

The closure is an "important measure to protect the public's safety and to ensure no-one finds themselves trapped in a park if fire breaks out in these extreme conditions," the Department of Planning, Industry and Environment said in a statement.

"Fire can move extremely fast through the landscape with new ignitions possible so every precaution to protect people is necessary."

Parks will only reopen when considered safe, the department said.

NSW Health also warned people with respiratory issues to take extra precautions as smoke from bushfires across the state are expected to again hit air quality levels over the coming days.

"People with conditions like asthma, emphysema and angina are more likely to be sensitive to the effects of smoke," Richard Broome, NSW Health Director of Environmental Health, said.

"Smoke from bushfires can be quite unpredictable, but we're expecting poor air quality in a number of places into next week."

Inspector Shepherd said fire crews have had little respite since the weekend: "The fires went gangbusters for a day but it didn't mean the fires stopped."

All it meant was fire-danger ratings had dropped back to "very high" fire danger, he said.

While resources will be strained in NSW, other states such as Queensland and Western Australia were also dealing with major blazes, he said.

"Even if we had 1000 more trucks and 5000 more firefighters, we couldn't guarantee having a truck near every home," Inspector Shepherd said.

While conditions will not be anywhere as dangerous on Monday as on Tuesday, the state a total fire ban has been declared because "the last thing we want now is to have more fires in the landscape", he said.

The Bureau of Meteorology's forecaster Abrar Shabren warned: "There is potential for conditions on Tuesday to be worse than they were on Friday [in Northern NSW]."

Greater Sydney could see temperatures soar to the high 30s, with winds of up to 50 kilometres per hour in the morning before shifting southerly to 40km/h in the evening.

Meanwhile, the Hunter region is expecting to see temperatures in the mid to high 30s, with winds reaching 45km/h in the morning before easing to 35km/h in the evening.

"Any wind is not great for bushfires. It can make bushfire fighting difficult and can worsen and irritate conditions," he said.

"There's no rainfall forecast for the foreseeable future - we'll have drier and hotter than average conditions for the next few months."