Power is finally beginning to return to some parts of Adelaide, after the entire of state of South Australia was crippled by blackouts.
SA Power Networks said electricity is slowly being restored to sections of the Adelaide metro area.
Meanwhile, the power outages have caused problems with the SA Water network, with some properties without water and left unable to clear sewage.
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It is believed lightning bolt struck a transmitter around 3.48pm ACST (4.18pm AEST), which affected a series of towers and lines in the Port Augusta region.
Electranet, which operates the high voltage transmission network, said electricity would eventually be progressively restored.
"We understand the impact of this event and are working hard to resolve it," the company said in a statement.
"At this point in time we don’t know how long it will take to restore power, but people should prepare for it to take at least several hours. SA Power Networks will then progressively restore supply to their customers as it is safe to do so."
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South Australia Police have declared the situation a Major Incident.
"The declaration has been made in response to the state-wide power outage that occurred this afternoon in conjunction with the severe weather events being experienced," a statement from police read.
"The community is asked to avoid any non-essential travel this evening.
"However, with [it] unclear how long full resumption of service will take ... the community being asked to remain calm, to avoid leaving their homes and to be patient on the roads if it is necessary to travel."
The Australian Energy Market Operator said the outage occurred when the network was disconnected from Victoria.
An estimated time when power will be fully restored is yet to be determined.
"The Australian Energy Market Operator (AEMO) can confirm that as at 16:18 AEDT on 28 September 2016, the South Australian electricity market disconnected from Victoria, resulting in a state-wide power outage in South Australia," a statement read.
"AEMO is working closely with transmission network service provider ElectraNet to identify and understand the severity of the fault, as well as determine a power restoration time."
Adelaide's CBD was plunged into darkness, with all traffic lights down and vehicles gridlocked.
Adelaide Airport, hospitals, the Adelaide Arcade and 9NEWS studios were among those left relying on backup generators.
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According to the SA Power Networks website, about 200,000 properties were without power.
"What we have is essentially a weather event which has damaged some infrastructure," Premier Jay Weatherill told ABC radio.
"The system protects itself and protects the assets by closing down because of the sudden surge in the system."
The severe weather system reached Adelaide at around midday local time, with 100mm of rain expected in the Adelaide Hills.
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 have already responded to more than 450 calls for help, most because of fallen trees or rising water.
Do you have images or video of the wild weather? If so, email contact@9news.com.au, but remember to stay safe.
Lightning struck a tree in Myrtle Bank, causing it to fall on a home on Auburn Avenue.
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Almost 18,000 State Emergency Service and firefighting workers and volunteers were placed on standby, with a team from Western Australia also called in to lend a hand.
The RSPCA also urged pet owners to look after their animals in the storms.
The Bureau of Meteorology said the extent of the low pressure system was unusual.
"This depth of the low, this close to the coast, is very damaging," BoM SA director John Nairn said.
"It's a very significant event for South Australia. It's very rare."
SA Police has urged drivers to be wary of floodwaters, particularly in The Mount Lofty Ranges.
Winds were expected to average about 90km/h, whipping up ocean waves to more than 10 metres, but gusts of up to 140km/h are possible for the Eyre Peninsula, according to Weatherzone.
"In terms of winds… and widespread lightning and thunder, it could be one of the worst storm systems in decades," Weatherzone senior meteorologist Jacob Cronje told 9news.com.au.
Flooding could be a problem, with rainfall of up to 20mm forecast for "everything east of the York Peninsula" up to midnight tonight and further falls of between 30mm and 40mm tomorrow, Mr Cronje said.
The winds and thunder should start to ease late tomorrow, but the rain will likely continue until Friday.
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For a full list of SA weather warnings, visit the BoM website.