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Three crews were called shortly before 6.30am to the abandoned Glamis Hospital in Montpellier St and were able to extinguish the blaze in about an hour.
Speaking from the scene this morning, Fire and Emergency New Zealand (Fenz) fire risk management officer Mark Bredenbeck said there was no question the fire had been deliberately started, given there was no electricity in the building and no electrical storms overnight.
The former hospital had sustained yet more damage after being gutted by another suspicious fire late last year and police had been notified.
Whoever set the fire had put themselves in grave danger, given the already dilapidated state of the building, Mr Bredenbeck said.
All residents and staff at the Glamis site were transferred to the Yvette Williams retirement village in Highgate when it opened in early 2011.
The building was well known as a hot-spot for break-ins and vandalism.