"It was like a bomb went off," said fish-and-chip store operator Dawn Leahy. "I was holding onto the front door of my shop watching things flying past."

The main street of Rutherglen is in disarray

Senior forecaster for the Bureau of Meteorology, Terry Ryan, said Thursday night’s tornado - there may have been more than one - had initially been assessed as between F1 and F2 on the scale for tornado strength, meaning they had wind speeds between 117 and 253 kilometres an hour and a path of destruction between 150 and 250 metres.

One empty building has crumbled, a car dealership has been left barely recognisable after a wall was completely torn away and the Tuileries winery has lost part of its roof. The number of other damaged properties is in the dozens.

VIDEO: Raw vision of the tornado

Parts of Rutherglen have had no power since 10.30pm Thursday.

At Barooga, post office operator Max Steward likened the tornado to a lawnmower moving through his property.

The State Emergency Service says a car was picked up and tossed onto its roof at Mulwala, close to where the tornado struck.

Spokesman Lachlan Quick said most of the injuries had come from the Denison caravan park in Mulwala, where damage was extensive.

‘‘People were pretty fortunate to get out of there,’’ he said. ‘‘It looks like people were taking shelter in their caravans.

‘‘It looks the weather will calm down for the next couple of days ... we’re just grateful it didn’t happen over the Easter weekend.’’

Paramedics treated 20 people after dozens of calls in Yarrawonga and Mulwala, when severe storms, flash flooding and the tornado came through about 8pm on Thursday. Paramedics said there were as many as 80 ‘‘walking wounded’’ in the affected areas.

A couple in their 70s were flown to hospital in Melbourne after they were injured when their motorhome was overturned in the wind. Two men in their 50s were flown to hospital, both in a critical condition with head injuries.

Mr Quick said the towns of Bundalong, Yarrawonga, Koonoomoo and Rutherglen were among the worst hit, with several buildings damaged and fallen trees.

Crews spent much of Friday morning clearing corrugated iron and removing debris from trees, he said, after working through dangerous conditions overnight.

VIDEO: Tornado 'flattened' caravan park

The Moira Shire Council has set up an emergency refuge centre for those affected.

Across the affected areas, the SES received 180 calls for help, mostly for building damage.

Storm damage also briefly forced the closure of the Hume Freeway near Benalla.

Several people were hit by flying debris, Ambulance Victoria emergency management manger Jon Byrne said.

He urged people who did not need to be in storm-affected areas to stay away so emergency services could mop up.

To form tornadoes a long-lasting thunderstorm is needed with strong updraft winds, Mr Ryan said. As the front collides with humid surface air it begins to spin off and rotate.

‘‘They [tornadoes] are really good at dissipating energy, the whole show is about dissipating energy,’’ Mr Ryan said.

Mr Ryan said bureau radar images had captured the thunderstorm beginning to rotate just north of Yarrawonga at about 7.30pm.

But he said it was difficult to capture tornadoes on radar because they typically last only short periods of time and it was hard to know how many there had been last night.