Victorians have been urged to “hunker down” as wild storms that have cut power to thousands in NSW and South Australia are expected to pummel parts of the state.

Ferocious storms, giant hail and wind gusts of more than 125km/h are forecast across parts of south-eastern Australia as a cold front collides with a warm and humid northerly airstream on Friday.

It follows severe weather, hail and some flash flooding across parts of South Australia on Thursday. There were wind gusts of more than 130km/h at Port Pirie and Roxby Downs, and a 36-millimetre deluge in one hour reported at Mount Horrocks.

⚠️ SEVERE THUNDERSTORM WARNING has been updated for parts of #Melbourne, with severe thunderstorms impacting multiple areas.

HEAVY RAINFALL is likely, and may lead to flash flooding.

Full details:https://t.co/eUdR3A536p pic.twitter.com/IAXvrDjoob

— Bureau of Meteorology, Victoria (@BOM_Vic) October 18, 2024

Structural damage to high-voltage transmission lines has also cut power to BHP’s Olympic Dam mine. The dam is expected to be out of operation for up to a week.

“ElectraNet crews are working to restore power as soon as possible, and we are working with them to better understand the scale of the impact and recovery timelines,” a BHP spokesman said.

“In the interim, back-up generation and business continuity plans are in place.”

Roxby Downs Council CEO Roy Blight said winds of “over 130km/h” had caused the damage.

“Yesterday was a very hot day. It was over 40 degrees, and became very windy late in the afternoon, and there was a pretty severe storm that came through about 4.30 and that’s when the power went out,” he told ABC Radio Adelaide.

“That was followed by a pretty severe sandstorm, and the sky went red for about a quarter of an hour, and that was followed by a short, sharp rain shower, which sort of settled the dust and the sand.

“It’s like being sandblasted if you’re out in it. It’s not like dust, it is actually gritty, and it hurts.”

Thousands of people remained without power in SA on Friday, with the outage forecast to last days.

In NSW, the entire outback city of Broken Hill is running on back-up generators after a wild storm – including what the weather bureau said was a possible tornado – on Wednesday night flattened some of the 220,000-volt powerlines that supply power to the city and surrounding communities, causing a widespread outage.

Power authorities say repairing the damage from the burst of high-speed winds and heavy hail could take weeks.

Senior Bureau of Meteorology forecaster Miriam Bradbury said the risk of thunderstorms would be widespread in the east, from southern Queensland through NSW, Victoria and into northern Tasmania. Severe storms were most likely on Friday across NSW’s central and south-west slopes, and in north-east Victoria.

“In these areas, we may see locally destructive winds … in excess of 125km/h, [and] we might also see giant-sized hail or intense rainfall,” Bradbury said.

“These storm hazards may lead to downed trees or branches, power outages, dangerous driving conditions with water or debris over roads, localised flash flooding and some property damage due to winds.

Crowds heading to this weekend’s MotoGP at Phillip Island, south-east of Melbourne, have been urged to reconsider travel plans.

“Try to hunker down, make sure you’re where you need to be by [Friday] morning or put your plans off through those high-risk areas,” Victorian SES commander David Baker said.

There’s also an increased risk of thunderstorm asthma for northern parts of Victoria.

‘Supercell’ storm warning

Friday’s warnings come after Casterton, in Victoria’s west, was struck on Wednesday by what some locals called worst storm they had seen.

“Everyone is in shock by the sheer ferocity of it,” Heidi Herbert from Herbert’s Bakery said.

“I’ve never heard a noise like it before. It was like a freight train but worse … I stood outside on the verandah and it was just deafening.

“We saw the clouds come in and the thunder and lightening but had no idea this was coming. It all happened so quickly, it caught everyone by surprise.”

The weather bureau said Casterton copped a 20-25-minute bucketing, with strong winds, five-centimetre hail and flash flooding.

“There was heavy rainfall of about 22 millimetres in 20 minutes … that doesn’t sound like heaps of rain but when it falls in 20 minutes that’s significant,” Bradbury said.

The Victorian State Emergency Service took more than 80 calls, 65 relating to building damage or flooding.

The supercell storm which knocked down several transmission towers last night south of Broken Hill. Supercells are likely again tomorrow in parts of northern VIC and inland NSW pic.twitter.com/2ngg30rleY

— Thomas Saunders (@TomSaundersABC) October 17, 2024

Flood watch warnings are possible in coming days, although the front causing the trouble is expected to move off the east coast early on Saturday, taking the most severe weather offshore.

“Southern Victoria, Tasmania and eastern NSW will see easing showers during Saturday, with gusty southerly winds,” Bradbury said.

“At times, it will be a little cool through southern Victoria, but it will remain mild elsewhere.”

There might also be storms in south-east Queensland and north-east NSW, although they are not expected to be severe.

-with AAP