A fire that halted production at Fonterra's Kauri site in Northland started in a powder evaporation building.

As a result of the fire one person was taken to hospital and four others checked by St John ambulance staff.

The alarm was raised about 7.30pm on Thursday and Fire and Emergency staff, police and St John ambulance staff were called to the site 12km north of Whangārei, on State Highway 1.

Fonterra spokeswoman Louise Kattera said the staff member was taken to hospital for some extra precautionary tests and they had been discharged by this morning.

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Four other staff members were checked on site by St John ambulance officers and did not need to be taken to hospital.

About 30 people were on site at the time and evacuated when the alarms sounded.

"The small fire was detected and put out quickly by operational staff at site. They were able to do this before the fire and ambulance services arrived which is a real credit to the team who worked together incredibly well to manage this incident," Kattera said.

How the fire started was being investigated today.

"We are investigating the root cause but can confirm that it wasn't a gas explosion nor an electrical fault," Kattera said

Production stopped for a couple of hours so the incident could be managed.

The plant had been scheduled for cleaning so that was carried out and began operating again about 1.30am today.