Millions of Aussies are at risk of enduring rolling blackouts this week, amid a heatwave sweeping through NSW and unexpected coal outages across the state.

A severe heatwave is threatening to knock out electricity across parts of New South Wales as millions of residents brace for more stifling heat.

Residents in two states have been warned electricity blackouts could leave them in the dark as a sweltering heatwave puts electricity reserves to the test.

Millions of Aussies could lose power this week as a heatwave grips the country - amid warnings wet and wild weather will smash 'every state and territory'. A total of 347 sites suffered blackouts during an unplanned outage in Sydney's CBD at 3pm on Tuesday,

New South Wales faces the threat of rolling blackouts over the next few days as the state's energy grid faces one of its biggest tests since the closure of the Liddell Power Station last year.

Australia's most populous state is facing an elevated risk of blackouts amid a forecast looming heatwave, the Australian Energy Market Operator (AEMO) warned on Monday.