Synoptic maps show the incredible ring-like low pressure system that has smashed the southern and eastern parts of Australia since Wednesday, pushing unprecedented winds up into South Australia and across to NSW, Victoria and Tasmania.

South Australia faced more rain and some of the strongest winds it had ever seen overnight with 120km/h winds on the Neptune Islands in the Spencer Gulf and 115km/h gusts recorded in Cummins on the Eyre Peninsula.

There are fears of flooding with many rivers expected to peak later on Friday.

Storms on Thursday followed a once-in-50-year event that hit on Wednesday, snapping electricity transmission poles like twigs and plunging the entire state into blackout.

The ring-like wind phenomenon hitting Australia since Wednesday, known as a cut-off low pressure system, has been unusually ferocious.

A cut-off system occurs when a low pressure system breaks away from the main flow of westerly winds that lie south of Australia. It brings heavy rainfall and gusty winds. 

"It's unusual for it to be this well organised and this well developed in Australia," Rob Sharp, a meteorologist with Weatherzone, said.

"It wraps all the way around with the associated cold front and the trough that continues up into Queensland. We often see cut-off low pressure systems but it's rare for them to be as slow moving as this one and as powerful as this one."

He said South Australia had suffered the double-whammy of a summer-like thunderstorm on Wednesday, with more than 100,000 lightning strikes, and then a winter-like storm on Thursday with cold temperatures and huge winds.

"In a sense, South Australia got the worst of the summer [storms] followed by one of the worst of winter storms this year," he said.

Many towns were hit with hail and flooding and the state declared a period of emergency.

"A line of severe storms over South Australia did produce large hail, and even a few confirmed tornadoes with locally destructive winds," Michael Logan, a meteorologist with the Bureau of Meteorology, said.

"The low pressure system was winding up in the Great Australian Bight, and in the evening and overnight period that tracked north, pretty much over the top of Adelaide."

About 20,000 South Australian residents were still without power on Friday morning and some have been told it may be days before it's back. 

Power outages had a devastating effect in many areas; Adelaide newspaper The Advertiser reported that embryos at a fertility clinic were destroyed when a back-up generator did not come on line for a few minutes after power was cut. 

A back-up generator also failed at the Flinders Medical Centre, forcing 17 patients in the intensive care unit to be moved to the adjacent private hospital.

By Friday night, the worst of the weather will have left South Australia but several towns are now facing the risk of flooding with further rain forecast for next week.

Mr Logan said the vigorous low pressure system would take a long time to dissipate.

"This system is very slow moving and actually the low will just slowly weaken almost as it slowly evolves through this period."

The intense low pressure system over central Victoria will slowly move south-east during Friday, directing vigorous, gusty winds across NSW, the bureau said.

The Hunter Valley will bear much of the brunt of the wild weather with winds of up to 90km/h and a gale warning for coastal areas.

Heavy rain will continue to fall across the South West Slopes on Friday, hitting towns that are already crippled by floods.

More than 300,000 litres of milk destined for supermarket shelveshas been tipped out by farmers in central western NSW as trucks struggle to get through flood-stricken areas.

On Thursday, the highest rainfall was in Batlow (60mm) and Long Plain (66mm) in the Snowy region. 

On Friday, the weather bureau said severe weather warnings were in place for the Hunter Valley region and small parts of the Mid North Coast, Central Tablelands and Northern Tablelands forecast districts stretching from Katoomba up to Tenterfield.

The bureau also said strong wind warnings were in place for Sydney Closed Waters, Byron Coast, Coffs Coast, Macquarie Coast, Sydney Coast, Illawarra Coast, Batemans Coast and Eden Coast.

Flood warnings were in place for the Lachlan, Macquarie, Bogan and Murrumbidgee rivers.

Weatherzone is owned by Fairfax Media, publisher of this website.