The entire state of South Australia was without power Wednesday after being pummelled by a "unprecedented" storm, with the blackout causing chaos as authorities warned of more wild weather.

The state -- about one and a half times the size of France and with a population of some 1.7 million -- lost power Wednesday afternoon after severe thunderstorms struck with destructive wind gusts of up to 140 km per hour (87 mph), torrential rain and large hailstones.

Trees were torn down, roofs ripped out, thousands of homes and business were without power and cars were gridlocked on flooded roads as traffic lights stopped working in what South Australia's Premier Jay Weatherill said was an "unprecedented weather event".

"We had winds which were so strong that when they hit power lines they were creating such energy, they were tearing the towers out of the ground," he told reporters.

"We had 80,000 lightning strikes that were hitting our generators. All of these events together have combined to bring our (power) system down."

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Mobile phone coverage was also disrupted and the loss of power at pumping stations was affecting water and sewerage systems, the Australian Broadcasting Corporation reported.

Weatherill said there were no reports so far of serious injuries or deaths but called on people to avoid travel as authorities sought to restore the network "in the coming hours", with power returning to some suburbs in the capital Adelaide late Wednesday.

He added that the grid had shut down to protect itself after a "dramatic drop in frequency" in the power network.

SA Power Networks, which runs the state's electricity distribution network, tweeted that South Australians should "brace for extended outages and ensure you conserve mobile device battery".

"We're experiencing a statewide outage and have no supply from the upstream transmission network," it added.

The stormy conditions were forecast to continue through Thursday, Bureau of Meteorology senior meteorologist James Taylor told AFP, bringing strong winds and persistent rain.

"It's definitely an unusual event to have such a deep low pressure system approaching the coast at this time of year," he said.

"Speaking broadly, it certainly seems to be one of the most intense systems we've seen for some time, with people not being able to remember anything like it in the last 10 to 15 years."

Federal Energy Minister Josh Frydenberg told Sky News "serious questions" would be raised about how a major state's entire electricity supply could be knocked out by severe weather.