Nine fire crews have been battling a blaze at the Pinepac warehouse in Whenuapai after two hoppers containing woodchip caught fire.
The fire service was called around 3pm by workers at the plant on Airport Rd. Nobody was hurt in the fire, which is contained in the hoppers outside the building.
Assistant area fire commander Vaughan Mackereth said there were nine fire trucks, two specialist aerial trucks and lighting appliances at the scene.
The main fire had been put out but crews were likely to be working for most of the night as the hoppers were proving difficult to access, he said.
The hoppers are around 12-14m tall and contain up to 250 cubic metres of woodchip.
They are used to feed timber onto a conveyer belt, and are open at the top with a small hole at the bottom. That means they can only be properly doused with water from above, Mackereth said.
"The trouble is we can't get into them," he said. "It's just smouldering, we've got it out - but we're going to be here most of the night."