Staff at the Norske Skog Tasman paper mill in Kawerau have described an explosion at the plant as one of the most serious incidents there in years.
The blast earlier on Tuesday evening was caused by a build-up of gas, a by-product of the manufacturing process, in one of the plant's receiving units.
It forced cladding off the building's exterior, and
It resulted in significant damage inside the building, and even blew cladding off the plant's exterior.
Staff were evacuated but no one was injured.
Kawerau station officer Neil Patrick says the scene is now deemed safe.
Mr Patrick says the plant's managers are carrying out an investigation.
More than a dozen Fire Service appliances were called in from as far away as Tauranga and Rotorua.