Three firefighters from the United States have been killed in the crash of a Lockheed C-130 Hercules air tanker in the Snowy Monaro area of New South Wales, where it was battling an aggressive bushfire.

"Reports are that there was a large fireball associated with the impact of the plane as it hit the ground," said NSW RFS Commissioner Shane Fitzsimmons.  "Our hearts are with all those that are suffering in what is the loss of three remarkable, well respected, crew that have invested so many decades of their life into firefighting."

Mr. Fitzsimmons said the cause is under investigation.  Conditions in the area the time if the crash were hot, dry, and windy with a large amount of smoke and dust in the air.  Coulson Aviation, the Canadian company that owns and operates the plane, grounded its large air tankers in NSW and Victoria pending a review, and out of respect to the families of the crew members, with the full support of Commissioner 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."

Coulson officials will soon depart their home base in British Columbia to come to Australia to assist the investigation.  

The C-130 is one of aviation's most enduring aircraft.  It was first put into service in 1954 as a cargo, troop transport, and medevac aircraft.  In the 1970s, some were converted into water bombers to fight forest fires.  Coulson developed a system to dump huge amounts of that red fire retardant out of the C-130 and the workhorse added another duty to its portfolio.

"Today is a stark and horrible reminder of the dangerous conditions that our volunteers and emergency services personnel across a number of agencies undertake on a daily basis," said NSW Premier Gladys Berejiklian.