Large crowds are gathering at Sydney beaches on Saturday amid a heatwave which is set to linger over the Harbour City until Tuesday.

Big crowds were already packed into the iconic Bondi Beach before midday with the mercury set to nudge the high 30s across parts of the city.

Residents have also been urged not to ignore COVID-19 restrictions with Waverley Council warning of the possibility of a spreading event.

Waverley mayor Paula Masselos said that council and police patrols would be stepped up during the heatwave.

Mr Masselos said also visitor access would be restricted as a last resort and implored beachgoers to observe social distancing rules.

"Australia Day is a time for celebration but I would be really disappointed if a new COVID-19 cluster were to emerge in Sydney because people ignored the Public Health Order limiting outdoor public gatherings to 30 people or the health advice about social distancing by 1.5 metres or keeping one towel-length apart," Mr Masselos said.

He also urged beachgoers to leave if beaches were too busy.

By 11am, the temperature had already hit above 30C in parts of western Sydney including Penrith, Badgerys Creek and Campbelltown.

The Bureau of Meteorology is forecasting Penrith and Richmond to both hit 37C on Saturday.

In Sydney's eastern, northern and southern suburbs, conditions will still be hot with Bondi, Cronulla and Mona Vale are expecting highs of 27C

After hot conditions hit on Friday, western Sydney is in the midst of what is expected to be a five-day run of temperatures above 35C.

Authorities have asked residents to be on high alert for health dangers amid the heatwave which is set to stretch into Australia Day in NSW.

Sunday is expected to be the peak of the hot conditions for South Australia, Victoria, Tasmania and the ACT but the heat will linger in NSW.

While temperatures are expected to ease across other parts of south-eastern Australia on Tuesday, a top of 40C is expected for Sydney on Australia Day.

Originally published as Huge crowds hit beaches amid heatwave