"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.