The Australian Energy Market Operator has issued an alert for a potential lack of power reserve in NSW for between 4pm and 7.30pm on Saturday, when power reserves are likely to be below the minimum buffer, potentially causing blackouts for residents.

About 1.30pm on Saturday, AGL Energy Limited announced it was preparing to curtail electricity use at the Tomago smelter, which consumes about 10-12 per cent of the state's energy.

"AGL has provided significant notice to Tomago to allow them to make the necessary arrangements to safely manage their plant and people. This procedure has been implemented previously without adverse effect," the company said in a statement.

"With temperatures reaching above 40 degrees and high humidity in NSW today, AGL anticipates that energy demand will be high."

AGL said the Bayswater Power Station was running at full capacity, and the Liddell Power Station was undergoing "unavoidable maintenance" due to boiler tube leaks. It had two of its four units running at full capacity early on Saturday afternoon.

The sizzling conditions have already pushed the state's energy network to the limit, with demand for power among the highest on record.

On Friday, NSW avoided load shedding of households in part by asking Tomago Aluminium to reduce power output by about 300 megawatts. That was roughly the shortage that prompted blackouts to about 90,000 households on Wednesday evening in South Australia.

Since 1991, power has only been curtailed to the Tomago smelter a total of four times, including Friday and Saturday, AGL said.

Total fire bans remain in place across NSW on Saturday, with fire danger reaching "severe" levels in the Hunter and four other inland regions of the state. For Sydney, the threat is deemed "very high,"  while "catastrophic" fire ratings were in place across areas of NSW.

Rural Fire Service (RFS) Commissioner Shane Fitzsimmons said the situation in NSW was as "bad as it gets" warning Sunday would see the "worst possible conditions".

"To put it simply they are off the old conventional scale," he said. "It is without precedent in NSW" 

Commissioner Fitzsimmons said "catastrophic" fire ratings had been issued only once before in NSW - in 2013 - since national standardised ratings were introduced in 2009. Sunday's catastrophic fire rating will stretch from Dubbo to Coonabarabran to Port Stephens. 

"This is an area three to five times larger than January 2013," he said, when more than 140 fires were burnt across NSW. "[Any fire] will consume whatever in its path," he said. 

At 3pm on Saturday, the RFS said 49 bushfires or grassfires were already burning across NSW, 17 of which were not contained.

At 10am 45 bush or grass fires in NSW. 14 not contained. Statewide total fire ban in force all weekend due to dangerous conditions. #NSWRFS

Deputy Commissioner Rob Rogers offered a blunt message to those considering visiting fire-prone areas: don't go.

"Go to the beach instead, do something else, particularly those people who may be thinking about camping. Plan it for another time," he said.

He said firefighters were expecting a particularly tough five-hour period on Sunday, before a southerly change swept through on Sunday evening and into Monday morning.

Saturday's top of 39 degrees, would make it Sydney's 11th day above 35 degrees this summer if achieved, extending the record broken on Friday.

The warning comes on a day the mercury is forecast to climb to a top of 39 degrees in the city and a sweltering 46 degrees in Penrith and Richmond. Penrith is a chance of notching up its hottest ever day, a record that presently stands at 46.5 degrees.

The abnormal heat could push the mercury to the state's hottest overall temperature on record for February on Saturday, reaching about eight degrees above normal.

In the state's west, the small township of Ivanhoe, population 200, is tipped to reach a stifling maximum of 48 degrees on Saturday. It was already well on its way to that figure just before 11.30am, when the mercury hit 45 degrees. 

If the temperature does hit the forecast peak in Ivanhoe, it would match the town's hottest ever temperature recorded, on February 15, 2004. That day claims the title of the highest ever February maximum of anywhere in NSW, a record that could fall on Saturday.

Menindee, Bourke and Wilcannia on Saturday are each forecast to hit a top of 47 degrees. It comes a day after Hay Airport reached 47.4 degrees, making it the third-hottest temperature recorded in February for NSW.

Overnight, White Cliffs in the far west of NSW posted a low of 34.2 degrees, the highest minimum recorded in NSW for any month, according to Bureau of Meteorology data.

Don Harwin, the new NSW energy minister, issued a statement late on Friday thanking households and businesses for heeding calls to curb power use. He also announced the formation of an Energy Security Taskforce to examine ways to improve readiness for similar future events.

The NSW Government is again calling on households to reduce their demand on the energy grid by switching off air conditioning except for occupied spaces, and setting the thermostat to 26 degrees.

A Department of Industry spokesman urged people to switch off unused appliances, including those in standby mode. Water pumps for swimming pools or spas should also not be used during the hotter parts of the day, when demand is high, while lights should be switched off where possible.

"It's still important that people do what they need to do to stay safe in the heat but where possible we ask that they consider their energy use carefully and only use essential equipment, particularly during the peak times between 3pm and 7pm," the spokesman said.

During Friday evening, wholesale prices in NSW peaked at their maximum level of $14,000 per megawatt-hour. Matt Howell, the chief executive of Tomago Aluminium, likened the price jump to turning up "at your local BP service station and paying $460 per litre of petrol".

On Friday, before the latest cuts were announced at the smelter, Mr Howell was already angered by AGL reducing power by more than AEMO was asking for to maintain the stability of the grid. He said AEMO was asking for each of its three potlines to be powered down by one hour each in turn, but AGL - its sole power supplier - demanded rolling cuts of 75 minutes each.

Mr Harwin said it was common for coal-fired power stations to have transient faults.

NSW Health issued an air pollution alert on Saturday for increased levels of ozone in the atmosphere in Sydney, which is likely to affect residents with respiratory problems.

Dr Ben Scalley, from NSW Health, urged people with respiratory conditions in Sydney to take precautions due to high ozone pollution, caused by car exhaust and industrial fumes. Ozone pollution is particularly bad on hot, still days, he said.

"Parents are advised to limit outside play for children with asthma," Dr Scalley said. "Ozone levels reach their peak around 7pm ... and tend to be lowest in the morning, so it's best to plan outdoor play in the morning when the day is cooler."

He urged asthma sufferers to follow their asthma action plans and take their relieving medication where necessary. If symptoms get worse, those affected should seek medical advice.

On the weather front, NSW could average eight degrees above average - recording about 43 degrees - at the peak of the heat.

The current record for February is 41.99 degrees. Reaching the all-time high would be a bit tougher - that figure stands at 44.06 degrees, set on January 14, 1939.

"For NSW, the area coverage of the heat is extremely rare, comparable with events such as the 1939 heatwave that were associated with the Black Friday bushfires in Victoria," Karl Braganza, head of climate monitoring at the Bureau of Meteorology, said, adding that the current record February day for the state was set in 2004.

Cricket NSW was among sporting groups to cancel all grades of the sport on Saturday, and junior and senior associations followed suit. Many schools had already alerted parents they could enjoy a lie-in on Saturday rather than encourage their tyros on the sports fields of the state.

"My main concern is that grade cricket does not have the infrastructure in place to safely monitor and manage heatstroke in what is essentially an amateur volunteer-run organisation," John Orchard, Cricket NSW's doctor, said.

The NSW Rugby League abandoned the start of its junior league competition and cancelled women's fixtures.

Racing NSW stewards have moved Randwick's Apollo Stakes meeting from Saturday to Monday because the heatwave conditions.

After representation from leading stables, stewards decided it was better to move the meeting rather than risk horse and rider safety in the heat on Saturday.

The Tropfest short film festival is being held in Parramatta Park on Saturday night, after moving from from its previous location of Centennial Park. Organisers have moved the event's junior festival, Trop Jr, from Parramatta Park to Event Cinemas at Westfield Parramatta to ensure the health and safety of those attending.