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Doc Twizel operations manager Karina Morrow said about 1900ha of Doc land was harmed, but the full extent of the damage would not be clear for weeks.
It was the largest area of conservation land to have burned.
Staff were unable to fully access the land yet, but they visited the Tarnbrae Track area on Wednesday, and all tussocks there were "burnt to a stump".
It was impossible to predict the number of animal deaths, but several dead trout were found in streams.
No native fish deaths had been reported yet, she said.
"Until we can get out there and have a look, we can’t really predict it.
"Tussocks and beech forest can take decades and decades and decades to get back to where it was," she said.
"It’s such a loss, because it was such a well-looked-after area and incredibly unique in terms of it [being] untouched — it’s pretty devastating to lose that."
Mrs Morrow said that she held grave concerns for the
rest of the land, which was home to native ecosystems.
"That stuff was further away from the heat of the fire in the village — I imagine the closer you get to the village, the worse it’ll be."
Tracks and huts were closed until further notice, and the rebuilding phase would take some time.
About 25 Doc staff from Twizel and Aoraki had been on site since Sunday and would continue working in the coming weeks.
One ranger lived in Ohau village and had to escape from the fire before returning to help fight it, Mrs Morrow said.
"To be able to go from a situation like that, to fighting the fire when you’ve been personally affected, is pretty incredible."
Environmental consultant Rob Young, of Twizel, who formerly worked for Doc for 34 years, said tussocks were home to insects and lizards, and birds fed off them.
"It’s a reasonably complex ecosystem in a lot of ways," Mr Young said.
If plant roots were not damaged in the fire, the plant had a good chance of rejuvenating reasonably quickly.
"We’re talking 10 to 15 years."
Fire and Emergency New Zealand scaled back its operations in the area yesterday, as the fire was well contained and fire breaks were at the perimeter.
Incident controller Rob Hands said extra crews from Otago, Canterbury and Southland arrived to assist and their help was gratefully accepted.
"We have also been fortunate enough to be assisted by several partners, including Department of Conservation personnel, heavy machinery operators and local pilots."
Fire crews had also found Milo, a pet kelpie who had been missing from the village since Sunday.
A cordon remained in place and six crews would be on the ground today.
- Kayla Hodge