The three firefighters who died after their plane crashed while battling raging bushfires in southern NSW have been remembered as "brave Americans".

Investigators will begin piecing together the events that caused the large aerial water tanker, known as Zeus, to crash in the Snowy Mountains region on Thursday afternoon.

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

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.”

Plane crash victims remembered as 'brave Americans' as audio emerged of a terrified call for help_1
Inset: The three firefighters have been remembered as 'brave Americans.' Photo: AAP

Panicked audio of an operator on the ground calling for help as he watched the plane crash has been broadcast by Nine News. 

"Fire comms…message red speak to your captain. Message this is red," a man could be heard saying.

The man says the word "crashed" before the audio cuts out briefly.

"Yeah, fire comms…it's just a ball of flames. Over."

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

"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.

"… Thank you to these three, and to all the brave firefighters from Australia and around the world. Your service and contribution are extraordinary. We are ever grateful."

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

"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 ATSB said in a statement.

NSW Premier Gladys Berejiklian said Thursday's lethal conditions showed the unprecedented fire season was "far from over".

"We can't thank people enough for continuing, not withstanding the conditions, to put their safety at risk to protect lives and property of others," she said.

In response to the tragedy the company has grounded their Large Air Tankers fleet as a precaution and as "a mark of respect",  RFS Commissioner Shane Fitzsimmons told reporters.

The grounding of the fleet will have an impact on aerial firefighting capacity, but the decision was understandable, RFS Commissioner Shane Fitzsimmons.

"It's absolutely warranted and I support them 100 per cent," he said.

"They are very mindful of the emotional and psychological effect that such a tragedy will have on the rest of their workforce, not just here in Australia but in North America or Canada."

Fire danger ratings are forecast to drop on Friday as milder weather conditions set in across NSW.

Authorities will contact the families of the plane crash victims before they release their names to the public.

The plane was owned and operated by Canada-based company Coulson Aviation and contracted to the RFS.

The company's owners are travelling to Australia and are expected to arrive later on Friday.

Several countries have been helping Australia throughout the bushfire crisis, including the United States, Canada, New Zealand, Singapore, Papua New Guinea and Japan.

Some countries have sent firefighters and planes while others have loan military troops to help operations on the ground. 

The death of the three  Americans brings the number of firefighters killed during this bushfire season to eight.

– With agencies