Audio has emerged of the terrified call for help made as the C-130 water bomber crashed in a ball of flames while fighting fires near the New South Wates Snowy Mountains yesterday afternoon.

The RFS lost contact with the large air tanker at about 1.30pm, with a panicked call coming in not long after that saying the plane had crashed.

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The audio of the call was broadcast by Nine News.

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"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."

The crash killed all three occupants on board, who were all US citizens.

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 in the next 24 hours.

"The aircraft had departed Richmond with a load of retardant and was on a firebombing mission," the company said in a statement.

"The accident is reported to be extensive and we are deeply saddened to confirm there were three fatalities."

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Authorities said the names of the victims wouldn't be released until their families had been informed. All three were US firefighters.

The cause of the crash will be investigated by the Australian Transport Safety Bureau with support from NSW Police.

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

While some countries have loaned firefighters and helicopters, others have sent troops to help support operations on the ground.

The three firefighters who were killed were US residents.

The US ambassador to Australia, Arthur B Culvahouse, said he was "deeply saddened by the tragic news".

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

"The families and friends of those who we have lost are in our thoughts and prayers. Thank you Australia for your sympathy and solidarity."

Foreign Minister Marise Payne says she has conveyed Australia's condolences to the US after the death of three American firefighters.

"We are mourning the three American firefighters who died in the Large Air Tanker crash in the Snowy Monaro area today," she said in a statement.

"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.

"I have conveyed Australia's condolences to the United States Ambassador, Arthur B Culvahouse.

"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."