Stewart has extensive knowledge of the customary uses of Fiordland, particularly customary fishing. He is the former chair of the Oraka-Aparima Rūnaka and the former Oraka-Aparima Rūnaka representative on the Board of Te Rūnanga o Ngāi Tahu.
Stewart is a Tāngata Tiaki, appointed by the Minister of Fisheries, in recognition of his particular expertise in local customary fishing management and resource issues. He is the Oraka Aparima Rūnaka representative on the Customary Fisheries Advisory Group and the Te Rūnanga o Ngāi Tahu representative to the Southland Conservation Board.
Stewart has been a member of the Guardians of Fiordland since its inception and in 2018 was awarded a Queens Service Medal for his services to conservation and Māori.