Millions of people on the east coast are being urged to prepare now for the wild weather, expected to within hours.

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.

A burst of heat will sweep across Australia’s southeastern states during the next six days, causing temperatures to soar in Sydney, Melbourne, Adelaide,

The heatwave baking much of eastern Australia will rapidly make way for conditions offering the “perfect ingredients” for thunderstorms later this week, raising risks of heavy rainfall and even flash floods.

We are tracking a building storm threat today in every single state and territory as the tropical climate of northern Australia pays the southeast a visit,” Ms Osborne said. “Into the afternoon, we are watching for the threat of severe thunderstorm activity.

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.

AEMO issued blackout risk warnings for the state of New South Wales, including Sydney, from Tuesday through to Thursday with electricity demand expected to spike due to hot weather.