A derelict psychiatric hospital building has gone up in flames in Nelson.

Fire crews found the old Ngawhatu Hospital building in Stoke well-alight, when they were called by police at around 1am.

They've been there since, putting out the fire and dampening down.

Two fire trucks are still there.

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Shift Manager Andrew Norris says the building is unoccupied and has no power, so the fire is being treated as suspicious.

Investigators will be visiting the scene today.