A Fiordland helicopter pilot has reported seeing a Moose in the Fiordland National Park – the first credible sighting in decades.

Ben Young has just begun working for Southern Lakes Helicopters and was doing a company familiarisation as a rear seat passenger on a scenic flight piloted by Matt Deaker on Saturday when he saw a cow moose standing in a small clearing.

"It shook its head and I saw the big ears."

The helicopter turned back for another look but it was gone.

It would be easy to dismiss the brief encounter as an optical illusion, except that Mr Young is extremely familiar with moose, having previously worked as a hunting guide in northern British Columbia, Canada.

Moose researcher and biologist Ken Tustin, who has been tracing the fortunes of the Fiordland moose since the early 1970s, interviewed Mr Young on Saturday and has no doubt that he was someone able to make a qualified observation, and that he did, indeed, see a moose.

"A live animal sighting is very special," he said. "We're pretty chuffed here"

Ironically, Mr Young was a non-believer when it came to moose in Fiordland.

"I was 100% adamant that I didn't believe there was any moose left in Fiordland. It's kind of bamboozled me," he said.

"I thought that man [Mr Tustin] was crazy, until last week... there was no way I thought I'd ever be having this conversation."

And, even after replaying the sighting in his head countless times to be certain, he's very clear about what he saw. A keen hunter, he had been deliberately looking out for deer and had seen about 15 on that flight, including one just minutes before, and it very obvious they were two quite different animals.

"I've seen plenty of moose in my day and I've seen plenty of red deer in my day."

"I know what I seen [sic]."

Adding to the credibility of the encounter was that the site, in an unnamed valley on the north side of Doubtful Sound, was very close to an area where Mr Tustin and his wife Marg have found recurring moose sign in the past. That, coupled with Mr Young's familiarity with the animal, made the sighting reliable enough to conclude that a remnant population of moose still exists in Fiordland 110 years after their release, Mr Tustin said.

"Right time, right place, right person," he said.

"We're thrilled to have a new record which brings moose presence up to 2020... very special, very humbling. I'm absolutely thrilled to bits."

The pair of them went back to the area on Sunday in a bid to locate back-up sign that might yield a forensic result but they failed to find anything. Mr Tustin said this was not surprising because the valley did not offer much in the way of food, so it was likely the moose was just passing through.

It's been a long time between drinks for Mr Tustin, whose last positive indications that moose still existed was in 2015. He returned to the Herrick Creek area for 12 days in November but again failed to find any recent sign.

A decade ago Mr Tustin estimated the Fiordland moose population to be somewhere in the vicinity of 25 to 40 animals. That number was likely to be "considerably less" now, due to the depletion of food. Whatever population was left would be fragile, he said.

"They'll be having a really tough time now with the way the high numbers of deer have been competing for forest food... it's a really hard life for an animal in the bush right now."

He said vegetation had suffered under the growing pressure from exploding deer populations, to the extent that the deer themselves were now showing evidence of struggling to find enough food to maintain condition.

"This is really refreshing that Ben saw one – wow! The chances are getting less and less."

Ten moose were released into the Fiordland National Park in 1910 with the intention of starting a trophy game population. Over the years Mr Tustin has collected accounts that suggest about 25 were shot by hunters, mainly through the 1950s, but few have been seen since. The most recent sighting was recorded by a tramper in a hut logbook in 2000 but that note had been unable to be verified.

Mr and Mrs Tustin have spent untold hours in the bush tracking moose and collecting evidence proving moose still exist, including two positive DNA hair samples. For many years they maintained a series of trail cameras set up in areas they believed moose annually passed through. On at least one occasion Mr Tustin believes he has been close enough to smell one, however he has never laid eyes on a moose in Fiordland. He has written two books on the history of moose in Fiordland and continues to seek out evidence of their existence, all of which he critically examines with a scientific scepticism to be sure the pieces add up.

As for Mr Young, he's found himself something of a reluctant hero in this story. However, his curiosity has been piqued enough that he is now reviewing his future hunting locations.

"I think it would be worth a trip in there to get the chance of seeing it again."