Emergency services were working to establish the cause of the fire, and police were speaking to friends and family of the deceased.
Mr Duncan was a well-known South Westland identity and a former Ngai Tahu-appointed trustee of the West Coast Development Trust.
He lived in the corrugated iron shed, where the mezzanine floor had been converted into a bach. The building was also used as a workshop and stored a quantity of salvaged wood and other flammable materials used by Mr Duncan to craft furniture.
He was nowhere to be seen when a male relative discovered the smouldering ruin when he returned from an overnight hunting trip.
Mr Duncan's vehicles were at the property and his dogs tied up.
The shed sits on Maori reserve land about 730m west of State Highway 6 on the Mai Mai straight, south of the Karangarua River Bridge.
Karangarua resident James Scott said the police presence in the neighbourhood was obvious this morning as the investigation into the fire and the whereabouts of Mr Duncan continued.
He described the missing man as a "real character" who was part of the fabric of South Westland and who particularly knew a lot about the area's history.
- Westport News