Lynda Hewer said one look out her kitchen window confirmed the building was engulfed in smoke and flames.
The hangar was built to the side of a private airstrip, and the building was about 100m long and 30m wide.
Hewer described "absolutely massive black smoke" and lots of loud bangs as the fire took hold.
Greymouth police confirmed this morning the building possibly held firearms and ammunition.
Hewer said from her look at the scene, the large building and a MD500 helicopter had been lost.
Another helicopter, a Robinson 44 - which had been used by police in the past couple of days in the search for the missing Greymouth fishermen after a fishing boat tragedy - had apparently not long left the hangar before the fire.
"They've lost their Barn Bay Fishing shed and I understand they've lost a 500," Hewer said.
"It's a big fire for a little place. It's a real shame."
Greenstone Helicopters also owns a Squirrel and another Robinson helicopter.
Hewer said the fire was a total surprise.
"The first I knew was when the phone went. It was the local policeman. I just looked out the window and there was all this black smoke."
Robson is currently overseas while his wife, Pat, was staying in Nelson but was returning to Haast today.
Hewer said a Neils Beach resident only known as Colin was lucky to escape the fire as the hut where he lived was feeling the heat right next to the workshop complex.
The man had apparently been oblivious to the fire until Robson's son Michael burst in, telling him to get out.
"Apparently he was on his computer, not knowing what was happening."
Haast fire volunteers had done everything they could but by 10am their only task was to dampen down the remains.
Hewer said it was lucky the fire crew had managed to stop fuel storage tanks at the site imploding, by wetting them down.
"The shed is a total loss. There's still a few fuel tanks still. They're okay. It could have been a lot worse."
A Fire Service inspector was flying to the scene this morning and expected to begin an immediate investigation in conjunction with police already on the ground.
- Greymouth Star