A corrosive acid spill closed a section of State Highway 2 near Awatoto for more than three hours this afternoon.

Traffic was diverted on the arterial route after corrosive acid was emitted from the chimney stack at Ravensdown fertiliser factory.

Fire communications shift manager Craig Dally said they were notified about the spill at 12.22pm and sent two fire crews to the scene.

Large quantities of a dried product had spilled on the road and firefighters blocked the surrounding drains before they sprayed the substance to dilute it.

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A Ravensdown spokesman said the corrosive acid, created through the manufacture of super phosphates, had sprayed onto the nearby train track and road.

"It's not explosive or flammable, it's just corrosive."

The plant went into shut-down mode as soon as the emission was noted, they said.

"There was a small amount of liquid emitted from that stack and when that happens we notify the regional council, and the regional council notifies the Ministry of Health, and the Ministry of Health notifies the Fire Service."

The spokesperson said no injuries had been reported and the plant would remain in shut-down until it was known how the emission happened.

"The guys did everything right, when they saw the emission taking place they closed down the whole plant, so there's no risk of anymore emission."

A police spokesperson said the road was closed after the spill and reopened at about 4pm.

A Hawke's Bay District Health Board spokesperson said a health protection officer completed a risk assessment with the fire service at the scene of the chemical spill.

There were no immediate or ongoing risks to the public, they said.