The account raises lots of questions in my feeble old mind, none of which are answered. It's safe to assume that the original owner, who, according to the story, left the place unoccupied all those years but lived in a house on a parcel created by subdividing that original one, very likely kept the place heated and/or air-conditioned, as the seasons required, and he must have let water run through the pipes on occasion. Or so I'd think anyway. And the roof must have been replaced at least once over those 50 years, and the paint would have needed refreshing at some point. Et cetera, et cetera.
I'm unfamiliar with the climate in that part of New Zealand, but I've never known of any place where structures are impervious to the elements. The sun bakes 'em, water rots 'em, wind erodes 'em. Out here in the maritime Northwest, if you leave a house (or even a room) unheated, you'll very likely soon have an impressive collection of mildew and mold. The place will literally rot away from within. Gotta replace the roof every coupla-three decades. Even concrete walks and driveways and foundations show signs of age in considerably fewer years than the house in question has been standing.
And somebody must have come through the place with a vacuum cleaner and dust rags every now and then. I can just imagine how sad it would be to maintain a place like that in the hopes that loved ones would come to occupy it at some point, but that some point never comes around.