NSW Ambulance Director Education Alan Morrison warned that dehydration and sunburn could lead to life-threatening situations.

He recommended avoiding the sun as much as possible over the weekend.

“People have a habit of under-estimating the heat and over-estimating their ability,” he said.

“Ideally, they should be planning ahead and staying out of the sun during the hottest part of the day.

“If this cannot be avoided, measures such as regularly drinking small amounts of water and indeed ensuring you have enough water to last the day, can mean the difference between remaining healthy and ending up in hospital.”

Chief Superintendent Morrison said the young, elderly and chronically ill were the groups most at risk for heat-related illness.

“If you are in a high risk group, the next few days are not the days to be doing gardening or any strenuous work either outside, or even inside,” he said.

“People working in excessively hot environments, such as in roof cavities, can suffer a rapid and extreme escalation of body temperature which can lead to instant death.”

People should limit their intake of alcohol, energy drinks, coffee and tea as they can cause dehydration.

Those who did not have an air-conditioner at home should try to spend time in an air-conditioned place like a shopping centre, library or cinema.

The symptoms of heat exhaustion include nausea and vomiting, fainting and dizziness, loss of appetite, weakness, and/or headache.

If any of these symptoms occur, lay the person down, apply cool wet towels around the neck and under the arms and have them drink cool fluids.

If the person does not respond to treatment or they have severe symptoms like confusion or collapse, call Triple Zero (000) immediately and ask for an ambulance.

Emergency Services Minister Troy Grant said urged people who had planned to visit national parks or forested areas in the coming three days to change their plans, and for motorists to avoid using back roads.

"Weather conditions will gradually get worse from today through to Sunday. The most catastrophic likely weather will be centred around the Hunter, and will run from east to west right through to the central west of NSW," he said.

"If you have activities or trips planned to national parks, to forest or areas that are prone to potential fire, we ask you to change your plans and reconsider your movements.

"Take care when you are travelling throughout the state to use main roads in preference to back roads."

NSW Rural Fire Service Commissioner Shane Fitzsimmons said NSW was in for a "very difficult next three days", with the worst conditions expected on Sunday.

"Catastrophic fire danger ratings are the worst conditions that we can experience when it comes to fire danger ratings. Typically that's where we see the worst of fire behaviour, the most aggressive and rapid spread of fire," he said.

"Under those sorts of conditions, it is unlikely, unless we are there very quickly and very early, that fires that take hold, we simply won't be able to suppress. Our focus turns very much to the saving of life, the saving of property and getting as many messages and warnings out as we can in light of the conditions."

He said firefighters had access to an additional two large air tankers, which would be positioned strategically around the state to respond to any blazes.

As Sydney and other centres toil through another heatwave, the number of heat-related emergencies is on the increase at the state's hospitals, NSW Health said.

The St Vincent de Paul Society, meanwhile, said people on low incomes and the homeless were particularly vulnerable during period of extreme heat.

"During these incredible periods of hot weather there are more casualties than any other disaster or emergency. Dehydration, exhaustion and strokes can directly and indirectly result in death," Jack de Groot, the society's chief executive, said.

"The sad reality is that low income earners have the highest energy costs because their homes are poorly insulated, and their fans are the cheapest to buy but are electricity guzzlers," he added.

Authorities have recommended people head to shopping centres, RSL clubs and other public places with air-conditioning if they can't stay cool at home.

Saturday's sports fields, meanwhile, will be quieter than usual as schools and other competitions cancelling matches.

A total fire ban will be in force across all of NSW for Saturday and Sunday, due to worsening weather conditions.

The latest forecast from the Bureau of Meteorology confirms fire weather conditions will continue to deteriorate across Saturday and Sunday.

NSW Rural Fire Service (NSW RFS) Commissioner Shane Fitzsimmons said a broader area is now expected to experience Catastrophic fire danger conditions.

“The heatwave conditions will strengthen over the weekend, with many areas experiencing temperatures of around 45 degrees,” Commissioner Fitzsimmons said.

“This will be coupled with strong and gusty winds, and low humidity.

“We are likely to see broad areas under Severe and Extreme fire danger ratings.

“On Sunday, Catastrophic conditions will extend from the Central West, across the tablelands to the Hunter region and out to the coast.

“These conditions are dangerous, and any fire that starts will be extremely difficult to control.

“Do not be in bush fire prone areas during Catastrophic conditions.

“Discuss with your family and use the time now to decide what you’ll do. Do not leave decisions until the last minute.”

During a total fire ban, no fires may be lit in the open and all fire permits are suspended.

Gas and electric barbeques may be used provided certain conditions are met.

Lighting a fire on a day of Total Fire Ban attracts an on the spot fine of $2200.

The penalty for tossing a lit cigarette doubles during a Total Fire Ban from $660 to $1320.

You can report people who throw lit cigarettes from a vehicle by going to the RFS website or calling the Bush Fire Information Line on 1800 679 737.

Total Fire Ban rules and a map of current Total Fire Ban areas are at www.rfs.nsw.gov.au.