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 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.”

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

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“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.

“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.

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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, known as Zeus, 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.

AAP