South Australia has been plunged into darkness by severe weather damage to energy infrastructure which forced a shutdown of the entire electricity network.

High winds are being blamed for bringing down transmission lines and a lightning strike for taking out a sub station near Port Augusta on Wednesday afternoon.

Authorities warn it could be well into Thursday before full power is restored.

Hospitals and other emergency services are still operating using back-up generators and can continue to for at least 72 hours.

The state’s energy generation assets remain intact and its interconnector with Victoria is operational.

However Premier Jay Weatherill says people across SA should brace for several hours without electricity despite some outer Adelaide suburbs having services restored by about 7pm.

Mr Weatherill said the system worked as it was designed to and rejected suggestions it was the result of SA’s high use of wind power or the decision to shutdown coal-fired power stations in the mid north.

“What happens is the system protects itself by tripping all the generators in the system and also by severing the interconnector with Victoria,” he told reporters.

“It then triggers a protocol about how we bring it back up. There are a series of back-up generators which gradually re-power the system.

“Once the system is balanced then power is restored.

“But the number of hours this may take is something that we don’t know.

“This would happen to any system anywhere in the world if they had a similar circumstance.”

The statewide blackout prompted police to urge people to stay off the roads with officers dispatched to direct gridlocked traffic at major intersections during the afternoon peak hour.

There were reports of a number of accidents and all suburban trains were cancelled.

Police Commissioner Grant Stevens said there were also reports of some people stuck in lifts.

As the drama continued the SA Power Networks website listed about 200,000 properties without power.

The blackout came as strong winds and heavy rain lashed parts of SA with the Bureau of Meteorology warning at one stage that super cell thunderstorms were possible across a wide area, including Adelaide.

It said the storms could produce wind gusts up to 140km/h along with heavy rain that could cause flash flooding.

The extreme weather reached the city about midday and was expected to dump up to 100mm of rain in some areas, including the Adelaide Hills, where a flood warning was in place.

Similar falls were possible in the mid north across Wednesday night and Thursday with the cyclonic conditions expected to whip up large swells along the state’s coast, producing 10-metre waves.

The State Emergency Service had responded to more than 450 calls for help, most because of fallen trees or rising water.

It earlier distributed more than 43,000 sandbags to residents concerned about flooding after severe weather just two weeks ago inundated 80 homes across Adelaide and the Mt Lofty Ranges.

The bureau said the wild weather was the result of a front and intense low-pressure system.

It said records suggested such a severe system was last reported across SA more than 50 years ago.