A building at a paintball park south of Auckland that's been gutted by fire will probably have to be demolished.

The blaze at Asylum Paintball in the former Kingseat Hospital isolation unit broke out just before 4 this morning.

The building was fenced off when it wasn't being used as a playing arena and just had some furniture and discarded items in it.

One of the owners - Alex Malikov - says he's devastated and it's come as a shock.

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He says firefighters have told him the building that's burnt down isn't going to be usable again.

Alex Malikov says the paintball park will be able to operate in other buildings on the site once fire crews allow access.

He says firefighters are using an overhead crane to dampen down hotspots.