A kangaroo jumps in a field amidst smoke from a bushfire in Snowy Valley on the outskirts of Cooma on Saturday. AFP photo

Electricity supplies in Australia's most populous state, including its largest city Sydney, were under threat on Saturday after bushfires took out two substations with authorities warning of rolling blackouts if conditions worsen.

The blazes have raged across New South Wales, and they brought down transmission lines in the state's south connecting with neighbouring Victoria, state energy minister Matt Kean tweeted.

Kean urged people to reduce unnecessary electricity use and ‘turn off pool pumps, lights in unoccupied rooms and avoid using washing machines and dishwashers’.

TransGrid chief executive Paul Italiano said the system was coping but ‘under stress’.

He warned the loss of another major power station could mean ‘load shedding’ power cuts to prevent the electricity grid from collapsing.

‘It is no longer operating as a single national electricity market and that has compromised the availability of energy to New South Wales,’ Italiano told national broadcaster ABC.

New South Wales, which is under a third state of emergency over the severe fire conditions, has a population of just under eight million, of whom around 65 per cent live in the greater Sydney region.

The fires have claimed the lives of 17 people in the state and burnt some 3.6 million hectares (36,000 square kilometres) -- an area larger than Belgium.

While bushfires are common in Australia's arid summers, climate change has pushed up land and sea temperatures and led to more extremely hot days and severe fire seasons.