Audio has emerged of the moment stunned ground staff called for help after witnessing an aerial water tanker carrying three American firefighters plummet to the ground and crash in southern NSW.

“Fire comms… message red speak to your captain. Message this is red,” the man on the ground says in the call to Rural Fire Service command.

The audio, aired on the Today show on Friday morning, briefly becomes inaudible before someone says: “Crashed.”

“Yeah fire comms... It's just a ball of flames... over,” the man finishes.

Three fire fighters have tragically been killed, their water-bombing aircraft crashing in New South Wales. #9Today pic.twitter.com/F2dprzAU0H — The Today Show (@TheTodayShow) January 23, 2020

All three firefighters died in the crash on Thursday afternoon and have been remembered as “brave Americans”.

Investigators will begin piecing together the events that caused the large C-130 aerial water tanker to crash in the Snowy Mountains region.

The plane smashed into the ground and exploded in a "large fireball", NSW Rural Fire Service Commissioner Shane Fitzsimmons said.

“It is a confronting and sobering reminder of the inherent risks associated with firefighting and we've seen all too often this season,” he said.

“Our hearts are with all those that are suffering what is the loss of three remarkable, well-respected crew that have invested so many decades of their life into firefighting.”

Mr Fitzsimmons said they were “extraordinary professionals”.

View photos The firefighters, from the United States, were flying a C-130 aircraft similar to the one pictured. Source: AAP More

Tributes flow for three firefighters

United States Ambassador Arthur Culvahouse said he was "deeply saddened" by the news.

"The brave Americans who died near Snowy Monaro died helping Australia in its time of need," he said in a statement.

"Thank you Australia for your sympathy and solidarity."

Foreign Affairs Minister Marise Payne paid tribute to the US firefighters and said she had passed on Australia's condolences to Mr Culvahouse.

View photos Footage from the ground in the aftermath of the crash. Source: Today via Twitter/ Byron Kaye More

"Our hearts go out to their loved ones. They were helping Australia, far from their own homes, an embodiment of the deep friendship between our two countries," Ms Payne said in a statement.

Investigators from the Australian Transport Safety Bureau will travel to the crash site to start collecting evidence.

"Should a critical safety issue be identified during the course of the investigation, the ATSB will immediately notify relevant stakeholders so appropriate and timely safety action can be taken," the bureau said in a statement.

Extreme conditions likely to have downed plane, expert says

Aviation expert Tim Collins told Today the plane was likely to have crashed due to the challenging conditions faced by the experienced crew.

“It's extremely turbulent. It's extremely complex to operate a large aircraft close to the ground and the crew would be working very hard,” he said.

Mr Collins said extreme turbulence had the ability to break up an aircraft mid air.

He said it was also possible the firefighters had become disorientated in thick smoke or the strong winds had forced them to lose control of the aircraft.

Crews from the US and Canada, who are wrapping up their deployment to the #nswfires, have held a minutes silence as a mark of respect for the three American crew members killed in yesterday’s Large Air Tanker crash. We thank these crews for their support. #nswrfs pic.twitter.com/UNCt9APUMA — NSW RFS (@NSWRFS) January 23, 2020

Story continues