Authorities are pleading for public help to avoid blackouts with electricity demand forecast to exceed supply as a record-breaking heatwave sets in across eastern Australia.

Paramedics were also reporting a spike in call-outs during the hotter weather, with people suffering from sunburn, dehydration and heat exhaustion as Sydney set a record for the number of summer days over 35 degrees and most of the city roasted in the 40s.

The threat of power shortages later on Friday remains real, with the Australian Energy Market Operator (AEMO) warning that forecast demand for 5-6pm would exceed supply by 189 megawatts. The agency has called for a "market response" and intervene "as required".

AEMO said in a separate statement it had advised generators and market participants of "potential electricity supply shortfalls" in NSW and ACT during the afternoon peak from about 3.30pm (AEDT) until 5.30pm.

Those shortages may trigger the need for "localised load shedding to rebalance the power system", or rolling outages, to protect the overall network "from long-term and or prolonged damage", the operator said.

Cuts begin

AGL, one of the biggest producers, confirmed to Fairfax Media that it had cut some of its supply to the Tomago smelter in the Hunter Valley. 

"To maintain the stability of the electricity system, AGL has agreed with AEMO that it will [reduce power to Tomago]," an AGL spokesman said. "If power to the smelter is not curtailed, schools, homes and other small businesses will suffer a loss of power at the peak periods of demand this afternoon."

Tomago will be losing part of its supply by 3.45pm AEDT, a company official said. Each of its three aluminium potlines will go down for 75 minutes in consecutive periods, so that one potline is off at any one time.

Matt Howell, chief executive of Tomago Aluminum, said the cuts to the potlines were unnecessarily long to maintain grid stability, with AEMO telling them that one hour per potline would suffice.

Longer shutdowns - which allow AGL to sell any excess power at steeply higher spot prices - would also put the viability of the potlines at risk. "You have to ask AGL, why are they doing this?" Mr Howell said.

The Australian Workers Union asked Prime Minister Malcolm Turnbull to intervene. 

"Tomago is in a precarious economic position and it cannot afford for AGL to cut power," Daniel Walton, Australian Workers' Union National Secretary, said. "For AGL to do so would be an unimaginable act of bastardy."

Hottest day in play

Saturday could also be the state's hottest February day on record as an approaching front drags hot air southwards from central Australia, the Bureau of Meteorology said in a statement. Schools are beginning to respond to the predicted heat by cancelling sports events.

Don Harwin, NSW's Energy Minister, called on residents to reduce electricity use, particularly during Friday's late afternoon peak when power demand may hit record highs.

"Where you can please do your best to save energy - turn up your aircon to 26 degrees, adjust fridge temperatures, switch off unused electrical appliances and turn off lights where it's safe to do so," Mr Harwin said, adding that the government was also looking to cut usage where it can.

Mr Harwin is at the Sydney offices of TransGrid, the operator and manager of the NSW high voltage transmission network, and plans to remain there through the afternoon peak, his spokesman said.

Overnight, the AEMO renewed its request for a "market response" for additional capacity for NSW and also called for more supply for Queensland for 4-5.30pm Friday, local time, implying that state won't be able to provide its southern neighbour in a crunch.

Electricity consumption is already high, with the record NSW demand of 14835 megawatts a chance of being exceeded later today. (See live chart generated by Global-Roam below.)

Scorching temperatures

South Australia bore the early brunt of the heat, with power shortages on Wednesday evening triggering load shedding that blacked out tens of thousands of homes for about half an hour. Those cuts also reignited debate over the role of renewable energy, although locally available gas-fired power went unused.

The state avoided a repeat of the outages on Thursday even as the mercury climbed to as high as 48.2 degrees at Tarcoola, matching South Australia's record for February. 

That heat is shifting slowly eastwards and will push large areas of NSW into the 40s for three days from Friday. Bourke in the state's north-west may notch the highest temperatures, with 47 forecast for Saturday.

For Sydney, a top of 38 degrees is predicted, with 35 degrees recorded at Observatory Hill just before 1pm. That reading was the city's 10th day above 35 degrees this summer, beating a record of nine such days set in 1895-96.

The tally may climb to 11 by Saturday, when 39 degrees is predicted for the city.

More hot days increase the likelihood that Sydney will post its hottest summer after last month was its warmest single month and the previous month the second-warmest December in data that goes back to 1858.

Many sites across the city have exceeded 40 degrees, such as Penrith with 44.8 degrees.

Fire risks

"Catastrophic" fire ratings, meanwhile, are likely for parts of NSW on Sunday. Such a threat level has been issued only once before in NSW - in 2013 - since national standardised ratings were introduced in 2009.

Friday's heat will place a "very high" fire threat in place for much of the state, as the mercury nudges 45 degrees or higher. (See bureau of chart below of predicted highs across south-eastern Australia.)

For Sydney and most of the coast except for the Greater Hunter region, the fire threat is rated as "high" on Friday.

Several southern districts of NSW have total fire bans in place on Friday. Hay Airport was the hottest place in the state, reaching 47.1 degrees at 2.20pm.

Hospital alert

Westmead Hospital took proactive steps to protect its patients on Friday, after its emergency department recorded a spike in possible heat-related presentations during the December and January heat waves.

Hospital general manager Andrew Newton said air-conditioning had been diverted from some clerical offices to clinical wards, and non-clinical staff were offered the opportunity to work from home to avoid travelling in the heat.

"Our patients are quite vulnerable during these hot days, so we need to make sure we keep them as comfortable as possible and make sure we have plenty of cold water available," Mr Newton said.

Over 1,100 people with possibly heat-related conditions presented to Westmead, one of the busiest emergency departments in the country, in December and January.

There had been no serious heat-related conditions admitted to the hospital so far on Friday, suggesting the public was heeding NSW Health's repeated warnings to stay hydrated and out of the sun.

NSW Ambulance Superintendent Mark Gibbs has urged the public to use commonsense over the next fews days of soaring temperatures to stay out of hospital.

Drinking plenty of water and make good use of shade, fans and air-conditioners can be the difference between remaining healthy and ending up in hospital," he said.

"Anyone can be affected by dehydration or heat exhaustion; however the very young, the elderly and those with pre-existing medical conditions are typically the most vulnerable."

"Besides taking care of yourself, try to keep an eye out for others as well. Check on your friends and family, and pets as well, said Superintendent Gibbs.

NSW Ambulance advises people to be alert to the symptoms of heat exhaustion which include nausea and vomiting, fainting and dizziness, loss of appetite, weakness and/or headache.

Travel warning

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.

Hospitals, homeless

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.

More than 1100 people presented to Westmead Hospital's emergency department with possible heat-related conditions in December and January alone.

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.

With Kate Aubusson