Central NSW will experience an extreme heatwave event, according to Bureau of Meteorology data on Monday, and also at parts of the southern border of NSW. The rest of the state is expecting a severe heatwave except for the coastal areas where it will be a "low-intensity heatwave".

People have been warned to take care and follow the four major steps to survive a heatwave: drink plenty of water, keep cool, take care of others and have a plan.

The "unusual" event will see the potential for temperatures in some far west areas near Wilcannia to soar even above 47 degrees on Friday and Saturday.

The all-time Australian heat record of 50.7 at Oodnadatta in South Australia could be broken in the heatwave event as a mass of extreme hot air moves from central Australia across South Australia and eventually into NSW and northern parts of Victoria.

The highest ever temperature recorded in NSW was 49.7 degrees at Menindee Post Office on January 10, 1939. The hottest December day was recorded at Brewarrina at 48.9 degrees on December 9 1912.

BOM forecaster Helen Reid said it was "possible" that many December heat records would be broken in the oncoming heatwave event.

Possible records that could be broken (supplied by Weatherzone):

Wagga Wagga's hottest day in 75 years if it gets to the expected 44-45 degrees on Friday and Saturday.

Cobar's hottest December day in 55 years on Saturday (46 expected).

Hottest December day for many places in NSW for 20-40 years.

Bourke may reach 45 degrees for three days running.

Most severe heatwave for most NSW areas since November 2009.

Canberra could break its all-time temperature record with 43 degrees forecast for Saturday (previous 42.2).

The temperature forecast ranges from Tuesday for most areas west of the divide is staggering for its breadth and relentless heat until Sunday. They are: Tuesday 38-42, Wednesday 39-43, Thursday 41-43 degrees, Friday 43-47 degrees and Saturday 38-45 degrees. Very few places west of the divide on Saturday will be less than 40 degrees. Even the cherry capital of Young on the normally cooler South-West Slopes will be hitting 45 degrees on Saturday.

There is some relief on the way, but not until early next week. Brett Dutschke,a forecaster for Weatherzone, said a weak change would move through for southern areas of the state possibly very late on the weekend, that could trigger some showers and thunderstorms on the ranges and slopes in the ensuing days leading up to Christmas.

He said the western plains would continue recording 40 plus degree days and there was little possibility of any significant change for the plains in the next two weeks. There could be another cool change on Boxing day affecting the eastern and southern parts of the state.

Coonamble district vet Jillian Kelly said cattle owners should monitor their stock during the heatwave, making sure they had access to shade and that the drinking water was refreshed.

Dr Kelly said it was important that large ruminants who were on a grain diet had some roughage added to reduce the heat build-up inside them. In feedlots, bicarbonate of soda could be added to the diet to help reduce the effect of internal heat from a high-grain diet.

"They need to make sure they have shade for their animals and plenty of fresh water, plenty of easy access on long troughs, keeping an eye on water temperature too as if it is too high they will not drink as much, so you may have to put some shade over the water or clean it out more frequently.

"While temperatures during the day will be high, the temperatures at night will not drop that far enough for them to cool down so they will accumulate heat. These ruminants on a high grain diet will see that heat build up and so it can be important to add some roughage and good hay to the diet.

"In many cases there could be a double whammy of a heatwave and heat inside the cow that could lead to pneumonia and possible death from heat stress. In feedlots bicarbonate of soda could be added, although before changing diets, consult an animal nutritionist," Dr Kelly said.

It was important to monitor the stock in the heatwave and also refrain from moving them, but if it was necessary, only do it in the early morning.

Meantime, at least five homes were lost on Sunday night when a backburn operation on the southern flank of the massive Gospers Mountain fire ran out of control on the Bells Line of Road affecting homes in the Mt Wilson and Bilpin areas.

The RFS is bracing for the heatwave conditions from Thursday and right through the weekend.

A spokesperson for the RFS said: "Fire danger is not forecast at this stage to be 'catastrophic' but it is certainly going to be horrendous conditions for the firefighters on the ground."

At least 2 million hectares of mainly National Park forests has already been burnt in NSW since fires erupted in September.

A NSW Health spokesperson said: "NSW Health is reminding people to be cautious during the extreme heatwave conditions that are forecast across much of the state from Wednesday.

"People are urged to stay indoors during the heat of the day, minimise physical activity and keep hydrated during the heatwave. With expected temperatures rising to over 40 degrees Celsius for much of the state until Saturday, NSW Health is encouraging everyone to take the risk of heat related illness seriously," the spokesperson said.

"Extreme temperatures can cause severe illness, hospital admission and even deaths.

"Heat puts a lot of strain on the body and can cause dehydration, heat stress and heat stroke. It can also make underlying health conditions worse. People over 75, people with chronic medical conditions and people who live alone are particularly vulnerable. With the hot conditions approaching, it's a really good time to get in touch with elderly neighbours, friends and relatives to see if there's anything you can do to assist them."

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