The Fire Service is treating a fire at the former Southdown freezing works in Auckland as suspicious, but may never be able to prove conclusively what started it.
The blaze at the derelict works in Penrose has been burning since 6pm on Saturday and has engulfed the ground, second, third and fourth floors.
Acting chief fire officer Wayne Highet says it is still too dangerous for firefighters to enter the building and they could be fighting the blaze on Tuesday or Wednesday.
Pockets of fire cannot be reached because the collapse of the roof in the centre of the building.
Investigators are treating the blaze as suspicious, having been unable to find any other plausible cause.
But they may never be able to prove conclusively what started the fire because of the extent of the damage.
Charity group Drug Arm has lost half of its fleet of mini-vans in the blaze.
Many businesses used the site, including Drug Arm, which helps people fight their drug abuse.
Spokesperson Lesley Murgitroyd says two of its mini-vans were parked inside the building, though the loss should not affect its summer operations because the group is providing only skeleton services over the holiday period.