Large swaths of Australia on Sunday sweltered through a heat wave as authorities warned of an elevated bushfire risk in an already high-risk fire season during an El Niño weather pattern.

The nation's weather forecaster had heat wave alerts in place for Australia's most populous state, New South Wales, the Australian Capital Territory, Northern Territory and Western Australia, warning temperatures in some parts of the country could exceed 40 degrees Celsius.

An El Niño is a climate pattern in which unusually warm Pacific Ocean temperatures are associated with phenomena such as cyclones, droughts, wildfires and heat waves.

The high in the west of Sydney, the capital of New South Wales, was forecast at 39 degrees Celsius, almost 10 degrees above the February mean, forecaster data showed.

Hot and dry conditions combined with gusty winds prompted the forecaster to issue "extreme fire danger" warnings for parts of Victoria and South Australia states.