NSW Emergency Services Minister David Elliott and NSW Rural Fire Service Commissioner Shane Fitzsimmons on Friday farewelled the 24 Canadian and eight US firefighters who have been helping NSW crews for the past 35 days.

Their farewell was described as "sweet sorrow" by Mr Elliott after the death of three US firefighters when their air tanker crashed while battling bushfires in southern NSW on Thursday.

A minute's silence was held during the farewell breakfast on Friday.

"In my house when we farewell guests after their stay there's a black hole when they go," Mr Elliott said at the farewell event in Sydney.

"That's what we're going to find with you guys."

Mr Elliott thanked the crews for their contribution in what will be remembered as one of the "darkest summers" in NSW.

He also promised the state would come to the aid of US and Canada if needed in the future.

Mr Fitzsimmons said the US and Canadian fire crews made a discernible difference in the firefighting effort.

"What you have been able to provide is some reprieve, some rest for crews who have been going for weeks and months," he said.

The fire chief said the death of the three US firefighters was a "confronting and sombre" reminder of the extent of this year's fire season.

"We will be forever indebted to the enormous contributions and ultimate sacrifice that's been paid by these extraordinary individuals," he said.

Alaska region fire management officer Chuck Russell said the extreme fire weather and behaviour in NSW was surprising for firefighters.

"The sheer scale on the fire landscape is certainly unprecedented," he told reporters.

"It's been surprising to some by how much fire is on the ground."

He said each visiting firefighter was glad to come to Australia's aid and grateful for the opportunity to help.

"The Australians have been in it for a marathon," he said.

"Anything we can do to relieve that burden."

Brook Chadwick, who arrived in Melbourne on Friday as part of a 41-strong firefighting contingent from the US, said he and his Victoria-based colleagues sent their condolences to the families of the three men.

"Firefighting is an international business as you're aware, and it is a very small community. It definitely impacts every one of us regardless of what country you were born or where they fight fire," Mr Chadwick told reporters.

"Every one of us feels the sadness from yesterday."

Australian Associated Press