Fire which swept across the rooftop of Ravensdown's storage plant in Severn St, Pandora, on Tuesday started in the high roofing structure and not a conveyor belt system as initially thought.
New Zealand Fire Service safety officer Bob Palmer said the cause of the blaze at this stage was unknown.
"But it was not the conveyor belt - it was up in the structure of the building."
He said specialist investigators had been called in to help determine what sparked the blaze, which broke out just after 11.30am and caused clouds of thick dark smoke to drift across the Pandora industrial area.
Mr Palmer said initial examinations of the damage did however indicate the cause appeared to be accidental.
The most pressing concern for Ravensdown was to get the damaged roofline quickly and temporarily repaired as several more days of rain were forecast for the region.
"We've been lucky to get this weather window," Site Engineers spokesman Winton Brogden said yesterday as crews began tearing off the charred roofing.
"There's a lot of valuable product in there so our job has been to pull the damaged top off and securely tarp it up to keep the weather out."
The temporary roofing would secure both the fertiliser product below as well as the fire-affected area so that insurance assessors could carry out their examinations.
"When that is all sorted out we'll be back to put the new roof on."
About 7000 tonnes of material including rock potash, ammonium sulphate and potassium was stored in separate bins inside the building.
Ravensdown communications manager Gareth Richards said the site's production storage area was not affected, and by working in with the Awatoto plant he was hopeful orders would not be badly affected. "It's about cleaning everything up now."
