​Health experts are advising the community to drink plenty of water and plan ahead to avoid the heat of the day, with temperatures set to soar over the weekend and continue into early next week.

Sydney's west is bracing for temperatures in the early forties tomorrow, possibly reaching 45 on Sunday at Penrith. Central West New South Wales and communities north towards the Mid-North Coast are also tipped to swelter. And because of the hot, still conditions, air quality will worsen with ozone levels predicted to be poor across Sydney’s west as well.

"The first weekend of 2018 is going to be extremely hot, hovering in the forties in Sydney's west on both days, so it's very important for people to plan ahead to stay cool and hydrated," Dr Ben Scalley, Director of Environmental Health, said.

"A NSW Health study published just last month found extreme heatwaves lead to a more than 10 per cent increase in both deaths and ambulance callouts. They put a lot of strain on the body and can cause dehydration, heat exhaustion and heat stroke."

"At times like this, it's a good idea to avoid strenuous physical activity in the middle of the day, drink plenty of water, and take precautions when out in the sun such as wearing sunscreen and protective clothing."

But it's not just the heat this weekend which has Dr Scalley concerned, with ozone pollution on the rise along with temperatures.

“Ozone levels are higher outdoors than indoors, so limiting time outside during the heat of the day and in the evening would help people to keep cool and to limit their exposure to ozone pollution.” Dr Scalley said.

“As ozone can irritate the lungs, people with asthma need to follow their Asthma Action Plan and take their relieving medication where necessary. If symptoms get worse, they need to seek medical advice.

Dr Scalley also reminded the community that children, the elderly, and those with chronic medical conditions are particularly vulnerable to the heat.

"It's always a good idea to check in on how they're coping with the heat, especially if they live alone," he said.

"Anyone showing severe signs of heat-related illness should seek urgent medical attention through their GP or the emergency department at their nearest hospital."