Christmas storms set for Sydney's west

Christmas Day's sunny conditions in Sydney could turn stormy as showers and thunderstorms are predicted for parts of the city's west.

A 30 per cent chance of showers and even the chance of a thunderstorm has been predicted for the west by the weather bureau while other parts of the city should should remain dry.

The Bureau of Meteorology says Christmas Day temperatures are expected to reach 27C in the city and 33C in the west.

Parts of the country's southeast will swelter on Christmas, with humidity in the region expected to approach or exceed December records.

A severe heatwave is due to cross parts of NSW, Victoria, South Australia and Tasmania, with the average Christmas night temperatures for Tasmania and Victoria forecast to be the warmest on record.

Bureau of Meteorology senior meteorologist Jenny Sturrock says a trough over Western Australia has contributed to high humidity levels in Australia's southeast, with "noticeably muggy" conditions.