The huge warm air mass that broke national records in recent days will edge eastwards, setting up Sydney for three days in a row above 30 degrees.

While the unusual late-season warmth will have many heading for the beach or other outdoor activities, light winds and hazard-reduction burning in the Blue Mountains mean pollution will build up in the Sydney basin. Air quality is expected to be poor for the city on Wednesday, the government said.

The city's April warmth includes a record for the month of 35.4 degrees on Monday and is part of a broad heatwave the likes of which have few precedents this late in the season.

Rival periods for south-eastern Australia include bouts in 1922, 1938 and 1986, Blair Trewin, senior climatologist with the Bureau of Meteorology said.