Plant operations are back to normal at a Napier Fertiliser plant, where yesterday a sulphur fire injured two firefighters, forced roads to close and residents to evacuate.

Yesterday, central fire service communications shift manager Mike Wanoa said they received a call at about 2.18pm from Ravensdown in Awatoto, advising them a building had caught fire.

Fire crews went to a "fifth alarm" and had 60 firefighters and 12 appliances fighting the blaze during its peak.

About 7.20pm yesterday the site was handed back from the Fire Service, and today all staff have returned to work and normal site operations have resumed.

"The isolated building where the fire occurred has no visual damage and will be assessed for any minor damage today, with a thorough examination to ensure it is safe for staff to enter," Ravensdown CEO Greg Campbell said.

The air in the building had been tested regularly overnight with readings indicating the air is back to normal with no detectable sulphur dioxide found. Personal would not be entering the building until the structure had been accessed and formally signed off.

"I'd like to acknowledge and thank the emergency services for their efforts in containing the fire and ensuring all personal were safe and damage to buildings was restricted," Mr Campbell said.

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"They also took control of the situation in communicating with the local community and ensuring their safety. Also our staff, who followed our emergency procedures, which we practice regularly."

The sulphuric acid plant continued to operate through the incident, but superphosphate manufacture ceased for the night shift and was now available again for production. There would be no impact of fertiliser supplies from the plant.

"A thorough investigation as to what occurred will commence today, and any actions required implemented," Mr Campbell said, "I would like to apologise to those affected in the Napier community."

Yesterday Hawke's Bay fire area commander Ken Cooper said the fire was contained by 6pm but crews stayed on site to decontaminate equipment.

He said the "smouldering fire" started in the conveyor belt in the second shed.

"The conveyor belt went right through the building so it caused the fire to spread and sulphur is a very difficult chemical to extinguish," Mr Cooper said.

Fire assistant area commander Nigel Hall said there was about 800 tons of sulphur in the building.

"We had to use low pressure deliveries to put out the fire, as there was a possibility of a dust explosion," Mr Hall said.

A Ravensdown spokeswoman said sulphur in its form state is not flammable but once it breaks down into a dust it can catch fire.

Mr Cooper said as you added water to the sulphur a sulphur dioxide gas was produced, which made it dangerous for all involved.

"We were tactical about it and used spray instead of jets, as either way we had to use water to get rid of the fire."

In the process two firefighters were injured and treated by St John's at the scene.

"The sulphur dioxide gas caused eye irritation for two of the crew but they were treated and have since been released back in to the station."

The hazardous nature of the fire forced roads to close and residents to evacuate for a number of hours.

Police safety team sergeant Greg Simmons said residents along McLeod Rd, Waitangi Rd and Awatoto Rd were cleared out.

Health officials had urged people living near Awatoto or downwind of the factory to close windows and doors and to turn off air-conditioning units during the fire.

Medical Officer of Health Nick Jones said smoke from the fire and the burning sulphur could have affected people who had breathing issues like asthma or bronchitis.

Dr Jones said anyone who experienced breathing difficulties should seek medical help.

Rush hour traffic between Hastings and Napier was also affected, as State Highway 2 was closed from Farndon Rd to Waitangi Rd during the event.

Police said traffic on Farndon Rd and State Highway Two had moved very slowly while traffic on Pakowhai Rd moved at "walking pace".

Mr Simmons said the closure was to allow fire and police facilities immediate access.

Police had also patrolled the cycleways to stop any cyclists from filtering in to the surrounding area.

Mr Hall said they didn't know what caused the fire but this product was susceptible to spontaneous ignition at low temperatures.

A fire investigator is working alongside the owners of the building to try and determine what happened.

A Ravensdown spokeswoman said staff were safe and they were aware of the impact it had on their neighbours and appreciated everyone's understanding during the incident.

"We will be in touch with further information as soon as we can," she said.