Senior official Damian Coutts wrote the trips cost the taxpayer "substantially" and admitted it would be difficult to sustain them in future. He said "experience" and NZ Qualifications Authority unit standards were plausible excuses for taking the trips.
Another document suggested the training could provide benefits to New Zealand in the event of a catastrophic major fire.
Coutts also wrote that the training was relevant as the department was often asked to provide firefighters for "extreme fire environments" to Australia, Canada and the US
However, Taxpayers' Union executive director Jordan Williams called the trips a "controlled-burning of taxpayers' hard-earned money".
"This was an excuse for a junket, not training that furthers New Zealand's conservation. They might as well have learned the didgeridoo."
The two trips cost Kiwi taxpayers at least $105,791 and Australia's Government covered additional costs. The deployments were organised in 2012 and again this year. It's understood there were 25 staff on the first trip and 22 on the second.
Despite the opposition — and Coutts' prediction of future difficulties organising the trips — a DoC spokesman told the Herald on Sunday that another trip was on the cards and March or April were the preferred months for burn-off activities.
"So it's likely that it will happen then and we're expecting to participate in that one, yes, if the Australians are willing to have us," he said.
Williams expressed shock "that DoC is even considering more trips to learn a practice that is of absolutely no use in New Zealand". DoC appear to justify the trips to Australia on the basis that their staff enjoy them, he said.
"It's not clear what New Zealand taxpayers received for the spending. At best, Australia got cheap labour to assist with their fire management."
The information emerges just days after DoC was slammed for spending $6,300 on a "PR stunt" after flying three ministers, a TV crew and photographer to the West Coast for a photo opportunity.
It followed an announcement the Government planned to pass a law allowing the recovery of high-value native timber blown over in Cyclone Ita two months ago.
Conservation Minister Nick Smith would not comment on the fire spending.