A young volunteer firefighter and soon-to-be father was killed when a "freakish weather event" lifted his fire truck off the ground, the NSW Rural Fire Service (RFS) says.

Key points: Samuel McPaul died when his truck was flipped by a "fire tornado"

Samuel McPaul died when his truck was flipped by a "fire tornado" His wife Megan is expecting their first child in early May

His wife Megan is expecting their first child in early May Three volunteer firefighters have now been killed in the past two weeks

Samuel McPaul, 28, died yesterday after his truck flipped in the Green Valley blaze in Jingellic, 70 kilometres east of Albury near the NSW-Victoria border.

NSW RFS district manager Superintendent Patrick Westwood said Mr McPaul was "doing everything right" when tragedy struck.

He and two others had been mustering cattle caught in a paddock on flat ground.

"The crew decided to move away from that area and, quite unexpectedly, very suddenly, they experienced extreme winds and what could only be described as a fire tornado that lifted the back of the truck, fully inverted it and landed it on its roof, trapping three people, three crew that is, and unfortunately, one of them fatally," Superintendent Westwood said.

"The driver was a veteran captain of 35 years-plus experience.

"He thought he was in the right spot — as he was, from what I can understand — and just this freakish weather event that would have to be seen to be believed. Even then, other veteran firefighters don't believe what they saw, [it] engulfed that vehicle with flame, fire, and strong winds and literally picked up an 8-tonne truck and flipped it over."

In a statement, the RFS said Mr McPaul died, the driver sustained minor burns and a third member received serious burns and was airlifted to The Alfred Hospital in Melbourne.

"Tragically … one of our firefighters, a much-loved and respected local firefighter from down near the Holbrook area, tragically lost his life," RFS Commissioner Shane Fitzsimmons said.

Flags are flying at half-mast for Mr McPaul today. ( Twitter: NSW RFS )

Mr McPaul had been married for 18 months.

"He leaves behind a beautiful wife, Megan, who is pregnant with their first child, that's due on May 4," Commissioner Fitzsimmons said.

"As you would expect, the family is grieving and it's been a very difficult night.

"It would be fair to say I don't even think the comprehension has set in of the enormity of their loss."

Bushfire created its own storm

Superintendent Westwood said the fire had "created its own weather system," known as a pyrocumulus cloud, leading up to the incident.

"That cloud became very unstable," he said.

"It had a height of around about 8,000 metres and unfortunately, it appears to have collapsed during yesterday's deteriorating weather, causing erratic fire behaviour and erratic winds at the foot of the fire in all directions."

He said crews were pulled back to safe ground, but Mr McPaul and his fellow volunteers were caught up in the weather system.

A second vehicle was also blown over in the "same extraordinary weather event".

"On that same fire, through the same weather event, we also saw another vehicle and a couple of firefighters enveloped by flames," Commissioner Fitzsimmons said.

He said two firefighters received face and airway burns.

One of them was sedated and both were flown to treatment at a Sydney hospital.

"As everyone would understand this is a very, very sad day for the NSW RFS family locally and across the state," Superintendent Westwood said.

RFS Commissioner Shane Fitzsimmons said it was a "very, very sad day". ( ABC News: Brendan Esposito )

'Sam was the best of us'

Superintendent Westwood said he, the RFS Commissioner and the service's senior chaplain had spent time overnight with Mr McPaul's family.

Glenhaven Rural Fire Brigade paid tribute to Mr McPaul. ( Facebook: Glenhaven Rural Fire Brigade )

RFS service flags are being flown at half-mast today.

"I cannot imagine the terrible sense of loss and grief that Sam's family are now feeling," Prime Minister Scott Morrison said this morning.

"I have spoken to Megan to extend my deepest sympathies and our love and support at this terrible time and express that same sentiment on behalf of the entire country.

"Sam McPaul was the best of us."

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 41 seconds 41 s Scott Morrison speaks about the death of Samuel McPaul

Deaths rock RFS

Three volunteer firefighters have now been killed in the past two weeks.

Geoffrey Keaton, 32, and Andrew O'Dwyer, 36, both fathers to young children, died when their truck rolled after being hit by a tree in Sydney's south-west.

The crash injured three other passengers, who were taken to Liverpool Hospital.

"Both of these men were very well-respected … they're a very community-focused brigade, they work together, socialise together," Commissioner Fitzsimmons said.