A large blaze overnight at the Ravensdown plant in Napier forced firefighters to return on Wednesday morning to dampen remaining hot spots.
The fire spread in a single building on the site in Awatoto just before midnight Tuesday.
Napier senior station officer Mike Peachey said it was a "significant sized" fire, but it was contained to one building.
"It looked pretty impressive to start with, but it was confined to one of the processing areas for making fertiliser," Peachey said.