New South Wales is next in line for the mega-storm that caused havoc across South Australia on Wednesday and plunged the entire state into blackout.

The "vigorous" low pressure system and associated cold front will move through western and central NSW and north-west Victoria on Thursday, bringing damaging winds, rain and isolated thunderstorms, the Bureau or Meteorology said on Thursday morning.

An intense low-pressure system is making its way east from Adelaide across north-west Victoria on Thursday.

"It will bring potentially damaging winds gusts for the Mallee," Bureau of Meteorology duty forecaster Peter Newham said.

The bureau has issued a severe weather warning for damaging winds averaging 35 to 50 km/h, with peak gusts of about 90 km/h, expected to lash the region from 9am before dissipating in the afternoon.

The damaging winds, averaging 40 to 60 km/h, with peak gusts of about 100 km/h, will continue in the north-east before clearing about 10am. The Alpine areas could see gusts of up to 120km/h.

Heavy rain, which may lead to flash flooding, is also expected in the already sodden region and far-eastern parts of the North Central and Northern Country regions.

Areas which may be affected include Mildura, Swan Hill, Wodonga, Wangaratta, Bright and Falls Creek.

Sydney and the rest of the NSW coastline are not out of the woods either, with meteorologists predicting thunderstorm warnings to be issued on Thursday afternoon. 

Although the severe thunderstorms have left South Australia, the tail-end of the low pressure system will continue to hit the state with winds of up to 140km/h until Saturday, mostly to areas north and west of Adelaide. Up to 100 millimetres of rain across the Adelaide Hills may cause flooding.

Flights are arriving and departing at Adelaide Airport but some are cancelled or delayed by up to 12 hours.

About 75,000 homes in the north of the state remained without power on Thursday morning.

"There has been catastrophic damage to our energy infrastructure up in the north of the state so it will take considerable time to restore that," Premier Jay Wetherill said.

Political recriminations have begun with federal Energy Minister Josh Frydenberg and other MPs questioning the state's increasing reliance on renewable energy despite assurances that the switch to cleaner energy sources was not to blame.

South Australia was hit by more than 100,000 lightning strikes on Wednesday, which knocked over 22 transmission poles.

It's possible that a tornado formed around Blyth in mid-north South Australia, said Rob Sharpe, meteorologist with Weatherzone.

"Evidence suggests there could have been a tornado because of the level of the destructive winds there," he said. "The thunderstorms yesterday were incredibly powerful."

Bureau meteorologist Andrea Peace said the heaviest rain falls in NSW the south-west slopes of NSW including Dubbo, Forbes, Parkes, Wagga Wagga and Cootamundra.

"The heaviest falls [are] expected in quite a short period of six to 12 hours [on Thursday]," she said.

The rain is likely to exacerbate floods that have already crippled NSW towns including Condoblin and Forbes, where residents are furious that their main access road could be cut off for up to six weeks.

"We still have 22 flood warnings through the very saturated parts of eastern Australia, and we may see some renewed and prolonged river rises there with this additional rainfall," Ms Peace said.

Forbes is expected to be inundated by a second peak of the Lachlan River at the same time as the town's weekend floodwaters reach downstream Condobolin.

With 30mm of rain expected on Thursday and 20mm on Friday, the SES predicts the high-water marks will occur next week.

At least 50,000 sandbags have been transported into the towns and all court sittings at courthouses in Forbes, Condobolin and Lake Cargelligo have been cancelled for next week.

Ms Peace said gusty northerly winds will move across the east of NSW on Thursday morning, becoming gale-force in high Alpine areas.

The wild weather is expected to last into Saturday as strong winds continue "feeding or wrapping around the low" as it moves south-east.

In NSW, the Bureau of Meteorology has issued a severe weather warning for people in the Central Tablelands, South West Slopes, Riverina, Lower Western, Northern Tablelands and parts of the Mid North Coast, Hunter, Southern Tablelands, Central West Slopes and Plains, Upper Western and Snowy Mountains as well as for the Australian Capital Territory forecast districts.

Conditions on Thursday are likely to include:

A warning has also been issued to sheep graziers across central and western NSW as lambs and sheep exposed to the cold temperatures, rain and showers may die.

Areas of regional Victoria already saturated from weeks of rain face more heavy downpours and damaging winds on Thursday.

The intense low pressure system is making its way eastward from Adelaide and will hit parts of north-west Victoria.

The bureau has issued a severe weather warning for damaging winds averaging 35 to 50 km/h, with peak gusts around 90 km/h, that are expected to lash the Mallee region from 9am before dissipating in the afternoon.

The north-east of the state has been battered with strong winds and heavy rain since the late afternoon on Wednesday that will continue on Thursday morning.

Weatherzone is owned by Fairfax Media.