Around 600 schools will be closed across NSW as 'catastrophic' fire conditions threaten highly-populated areas around Sydney, Wollongong and Newcastle.

Childcare centres in high-risk areas will also close, with parents being urged to contact centres directly for more information. Some TAFE campuses will also shut for the day.

Early on Monday, Premier Gladys Berejiklian declared a state of emergency for NSW for the next seven days.

The NSW Rural Fire Service (RFS) warned that firefighters will not be able to reach many people in emergency situations if a fire takes hold.

"Under these conditions, some fires may start and spread so quickly there is little time for a warning, so do not wait and see," an RFS statement said.

"There are simply not enough fire trucks for every house. If you call for help, you may not get it.

"Do not expect a fire truck. Do not expect a knock on the door. Do not expect a phone call."

Which schools are closing?

More than 50 schools in bushfire-affected areas in northern NSW were closed on Monday, with hundreds more across the state to close tomorrow, according to deputy secretary of school operations, Murat Dizdar.

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"Some of those areas aren't receiving direct bushfire threat today but given what's imminent in weather conditions tomorrow we've decided to be proactive and make sure that the safety of our students, our staff, our communities is at the forefront of what we undertake tomorrow," Mr Dizdar said.

He asked parents whose children's schools were not closed tomorrow to "err on the side of caution" and make their own decisions whether their children should still attend.

"Simply make contact with your public school and let them know why your child won't be attending. I know that information will we be well received."

The NSW Department of Education has published a full list of sites that will be closed — this number is expected to rise as authorities continue to carry out risk assessments.

Included in the list are more than 100 independent schools.

In Sydney, some of the suburban schools earmarked for closure include Manly Vale Public School, Menai High School, Oatley West Public School and Lindfield Learning Village.

A firefighter defends a property in Torrington, near Glen Innes on Sunday. ( AAP: Dan Peled )

In the Blue Mountains the department is shutting down a number of schools including Katoomba High School and Leura Public School, while on the south coast and in the Illawarra, Mount Kembla Public School and Vincentia High School are among the schools closing.

On the Central Coast, Avoca Beach Public School and Blue Haven High School will be among those that shut down for the day.

Why should people in the suburbs be worried?

While many of the schools that are closing are in suburban areas, the NSW Rural Fire Service (RFS) is warning that the danger is real and that live embers could fly up to 30 kilometres from the fire front.

To give you an idea of the seriousness of the situation, on Friday, when more than 150 properties were destroyed in the state's north, embers were only travelling 12 kilometres from the front.

Bushfire behaviour expert Dr Trent Penman said the volatile wind conditions posed a clear risk for city and town properties, as they were geographically surrounded by forests and grassland.

"A single ember can start a fire in the bark mulch or straw mulch that someone has in their garden … which very quickly could move to the area around the house," he said.

"I think it is important not to be complacent and to realise that the risk is not just in the forest but in surrounding areas as well."

How do people get around?

Transport for NSW is warning people to avoid any non-essential travel on roads and public transport.

"If you need to travel, be prepared for conditions to change quickly and roads to close suddenly," it has advised.

With many schools closing, bus services to those sites would be cancelled as well, Transport for NSW said.

"If you need to travel, also make sure you've got water, food, supplies and a full tank of fuel."

Where should people go?

The RFS advised residents to leave their homes if they were even a little bit unprepared.

"The simple message will be, if you or your property are not prepared, you should not be in the area," Deputy Commissioner Rob Rogers said.

The RFS will publish a list of safe areas for people to take refuge from the risky fire regions.

"We are preparing maps that we will end up showing people to say where we think these fires will end up, and then there will be some specific advice for people within that potential fire path of what we will suggest they do," the Deputy Commissioner said.

The RFS suggests that people in risky areas, like on top of a ridge, or places with only one escape route should aim to be somewhere else in the heat of the day.

"Just don't put yourself in a bushfire prone area [like] right in the bush," Deputy Commissioner Rogers said.

"Don't be there for that part of the day if you are in that catastrophic fire danger area — it doesn't mean evacuate areas — it means think smart, be smart in your planning."

What about the smoke?

If you're not in a high-risk area but are affected by smoke, it is best to stay inside, with doors and windows shut and air-conditioning on to help filter the smoke, NSW Health has advised.

If you do need to travel, make sure you have up-to-date medication, especially for asthma.

If you don't have a prescription, you can still buy asthma relievers over the counter at the pharmacy.