More than 1 million people headed to beaches across NSW as high temperatures and ozone pollution hit Sydney and other parts of the state.

About 30,000 beachgoers visited Bondi, Surf Life Saving NSW spokeswoman Donna Wishart said.

"It is heating up out west so we are probably going to see a larger than usual influx of people from western Sydney heading to the coast, so that will put added pressure on," she said.

"It is one of the busiest days of the year and, statewide in previous years, we have had upwards of 1 million people on the beaches.

"Go to a patrolled location. There are plenty of other beaches that are open, so it is not worth risking, especially after the drownings nationally in the last few days," Ms Wishart said.

Bronte, Tamarama and Maroubra beaches were closed on Wednesday morning due to east and south-easterly swells. People, especially tourists, were warned to avoid swimming at these beaches. By late afternoon sections of Marouba and Bronte had reopened.

The NSW Office of Environmental and Heritage issued an alert at 8am yesterday for "poor" air quality across the Sydney region, with ozone levels exceeding national air standard qualities.

"In particular on very hot and calm days like we are seeing today, we see a big build-up of pollutants that form ozone in the Sydney Basin," the office’s director of climate and atmospheric science Matthew Riley said.

NSW Health executive director Jeremy McAnulty said people with respiratory problems needed to be careful.

"As ozone can irritate the lungs, people with asthma need to follow their Asthma Action Plan and take their relieving medication where necessary," Dr McAnulty said.

Extreme, record-breaking heatwave conditions are forecast to sweep across four states over coming days, sparking health and fire warnings.

A broad area stretching across much of southern Australia is set to experience the hot weather, with temperatures generally 10 to 14 degrees higher than usual for this time of year, the Bureau of Meteorology said.

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