Gigantic hail smashed Australia and an entire state was plunged into darkness yesterday afternoon as the "worst storm in decades" struck the country with force.

The violent weather caused a blackout the whole of South Australia, with traffic lights out of action and trams and trains cancelled, making travel virtually impossible.

Damage to infrastructure near Port Augusta at 3.48pm forced the whole electricity network to shut down, according to SA Premier Jay Weatherill. He said there did not appear to be any damage to the interconnector with Victoria and that the Australian Energy Market Operator was working to get the power back on.

SA Power Networks website said around 200,000 properties were without power, with the blackout not expected to be completely over until later last night.

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By 7pm (local time) power had started to be restored to some suburbs, mostly the metropolitan area's eastern districts.

There was a brief lull in South Australia, but an intense low-pressure system moves closer this morning, bringing with it gale-force winds of up to 120km/h and plenty more rain.

Meanwhile, trees were blown down and there were reports of some flooding in Mildura as strong winds from the storm entered Victoria about 10pm on Wednesday.

Heavy rain would follow for the next two days and emergency services would closely monitor North East and North West Victoria still swamped by recent floods.

Last night Victoria's State Control Centre duty officer Brad Dalgleish said, "We are seeing winds around 70km/h around the Grampians."

Melbourne was expected to get winds about half that speed, but gusts of up to 110km/h were expected in high places such as Mt Buller.

Streets flooded in Whyalla, South Australia. Photo / Facebook
Streets flooded in Whyalla, South Australia. Photo / Facebook

The wind probably wouldn't drop until 6am (AEST today).

"We are not expecting that it's going to be as extreme as what's happened in South Australia," he said.

But he urged residents to be prepared and be vigilant.

Roofs were blown off homes and power lines and trees came crashing down.

The state capital's roads were totally gridlocked as police patrols were dispatched to guide traffic at intersections and residents were warned to avoid travel wherever possible.

Police called on all people to avoid travel last night, with the blackout causing widespread traffic disruptions, especially in Adelaide.

Winds over 80km/h brought down a power line on to a car in Port Augusta, SA. Photo / Lauren Rose, Southern Cross, Twitter
Winds over 80km/h brought down a power line on to a car in Port Augusta, SA. Photo / Lauren Rose, Southern Cross, Twitter

A few buses were made available for some commuters. Hospitals and some other buildings were operating as normal on back-up power.

The blackout came as torrential rain and winds of 87km/h lashed the state, hitting Adelaide at 12pm. Facebook user Julie Minge shared extraordinary footage of golfball-sized lumps of ice battering her neighbourhood in Cleve, SA.

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Posted by Julie Minge on Tuesday, 27 September 2016

The Bureau of Meteorology warned super-cell thunderstorms across the central and mid-north districts could produce destructive 140 km/h gusts, heavy rainfall, flash flooding and more massive hailstones.

And the storm is coming for the rest of Australia next, with Victoria, NSW and Tasmania due to cop a deluge on top of massive rainfall recorded earlier this month, and parts of Queensland also affected.

Adelaide-based Kiwi Byron Gardner said the weather had been rough the whole afternoon.

"Light showers that started around 11am turned into heavy rain by early afternoon with strong wind gusts and surface flooding.

"The power switched off in my second floor office around 3:45pm in Adelaide CBD which overlooks the Adelaide parklands. Getting home was difficult due to all traffic lights being out and many roads were impassable due to flooding."

Travellers were stranded at Adealide Airport after power cut grounded planes. Photo / Tricia Watkinson, News Corp Australia
Travellers were stranded at Adealide Airport after power cut grounded planes. Photo / Tricia Watkinson, News Corp Australia

SA Ambulance asked South Australians to only call Triple 000 in a genuine life threatening emergency and SA Power advised people to conserve their mobile device batteries and prepare for extended outages.

The State Emergency Service earlier advised people to keep their phones charged up in case of power cuts.

Adelaide Airport said flights had been disrupted by the storm and told passengers to check with their airline.

The BoM issued a severe thunderstorm warning for an area stretching from Victor Harbor, south of Adelaide, to Marree.

Major centres to feel the impact of the storms included Whyalla, Port Augusta, Hawker, Port Pirie, Clare, Roxby Downs and Leigh Creek. The extreme weather reached Adelaide around 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 with the cyclonic conditions expected to whip up large swells along the state's coastal waters, producing 10-metre waves.

Lightning strikes a tree outside Gary Altman's home in Myrtle Bank, Adelaide. Photo / Tait Schmaal, News Corp Australia
Lightning strikes a tree outside Gary Altman's home in Myrtle Bank, Adelaide. Photo / Tait Schmaal, News Corp Australia

By late afternoon yesterday the State Emergency Service had responded to more than 330 calls for help, most because of fallen trees or rising water. It earlier distributed more than 43,000 sandbags to local residents concerned about possible flooding after severe weather just two weeks ago flooded 80 homes across Adelaide and the Mt Lofty Ranges.

SA fire services warned the public to ensure all heating, cooking and other appliances in use when power went out are switched off.

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

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

Thunderstorms are predicted to cause further river rises and the BoM has issued several weather warnings and flood alerts for areas already affected by the heaviest rainfalls in years.

The BoM warned the wild weather was the result of a low and a cold front which would affect Australia throughout the week.

It has gale-force wind warnings in place for the coastal waters extending from Perth all the way across to Adelaide.

A low is expected to develop further today which will bring huge winds to the South Australian coast. It comes from an unusually deep low pressure system forecast to move up over Kangaroo Island and into the Adelaide area this evening.

The low is expected to hang around for most of the week, bringing more wild weather across the country.

John Nairn told AAP the weather had the potential to cause widespread destruction. "This depth of low, this close to the coast, is very damaging," he said. "It's a very significant event for South Australia. It's very rare."

Adelaide can expect cooler temperatures as the cold front hits with a low of just 7C on Saturday before further rain hits on Sunday with a top of between 16-18C.

Winds over 80km/h brought down a power line on to a car in Port Augusta, SA. Photo / Lauren Rose, Southern Cross, Twitter
Winds over 80km/h brought down a power line on to a car in Port Augusta, SA. Photo / Lauren Rose, Southern Cross, Twitter

Victoria

Flooded parts of northern Victoria could see more stream rises with heavy rain forecast for today and tomorrow.

Thunderstorms and wild winds are also on the cards.

A severe weather warning is in place for large parts of the state, with a moderate flood warning also posted for the Loddon River.

"Most of the west and central parts of the state will see rainfall and that will continue into Thursday," Bureau of Meteorology forecaster Beren Bradshaw said.

A band of bad weather is moving across from South Australia and the rain will start to hit northern Victoria.

"There's a good chance that we will see renewed stream rises," Ms Bradshaw said.

Residents have been warned to watch out for heavy rain leading to flash flooding across the Mallee and Wimmera districts, with rainfall totals of 10mm to 25mm likely.

The mercury will plunge to a low of 8C on Friday, hitting a high of 23C on Sunday with showers predicted up until Tuesday.

New South Wales

Sydney is expected to avoid the worst of the wild weather, but central NSW has been warned to prepare for another battering.

The BoM has warned between 40mm-100m of rain is expected in the region from now until Friday, with the flood-hit town of Forbes expected to be hit hard.

The flood-hit Lachlan River is also predicted to peak next week near Condobolin and Euabalong, with 10 other flood warnings remaining in place.

Sydney can expect rain to set in today.

It's expected to clear bringing highs of 27C and 28C on Monday.

Tasmania

The BoM has predicted the island state is set to cop a drenching of up to 100mm as a "troublesome low" moves in.

Snow is also forecast in areas above 1000m while Hobart is bracing for heavy rain today.

Further rain and showers are predicted across the weekend and up until Tuesday with lows of 8C and highs of 17-19C.

Queensland

The sunshine state is also expected to be hit by the southern storm with the BoM issuing flood warnings for 11 rivers including the Dawson, Balone and Moonie.

Brisbane will hit a high of 28C today before showers and a gusty storm will hit today thanks to a trough and cold front moving through.

The wild conditions aren't expected to last long though.

The city will remain fine and sunny for the weekend with tops of 27-28C before further showers are predicted for Monday and Tuesday.

Thousands of homes were sandbagged in preparation for super-cell storms, heavy rain and possible flash flooding. Photo / Bianca De Marchi, News Corp Australia
Thousands of homes were sandbagged in preparation for super-cell storms, heavy rain and possible flash flooding. Photo / Bianca De Marchi, News Corp Australia

Western Australia

A cold front moved across Western Australia yesterday bringing showers to many areas including Perth.

The BoM has also warned a severe thunderstorm is expected for Esperance and Eucla.

The mercury is expected to plummet to a low of just 5C.

Showers and tops of between 18-20C are expected for the rest of the week.

A gale force warning is also in place for the Albany, Esperance and Eucla Coast.

Northern Territory

The Top End isn't immune from the wild weather with thunderstorms hitting Darwin from now until next Tuesday.

Temperatures are likely to hover around 34C, but no weather warnings remain in place.