Fire razed the category 2 historic Mount Aurum Homestead in the early hours of New Year's Day.
Police said three groups were at the Department of Conservation campsite area in the hours before the fire.
They have spoken to two but Detective Sergeant Derek Shaw said they are still seeking the third group - a couple in a maroon-coloured vehicle.
Mr Shaw said the woman is believed to be from Queenstown, while the man may be transient across the South Island.
Police said they believe they know his identity and want him to come forward.
The couple were not present when the fire was noticed around 2am.
The cause of the fire is unknown at this stage, but police said it is being treated as suspicious.