Residents of Wairoa have been told to stay inside and shut their windows amid an ammonia leak from the local freezing works.
Fire and Emergency NZ issued the warning at 8.47am, calling it a "hazardous substance emergency".
"Please stay inside, close all windows and doors and wait for further instructions", the alert said.
The leak, at Affco Wairoa, was low-risk "at this stage" but the measures were being taken as a precaution.
Affco's meat works plant is sited in the centre of the town, in the north of Hawke's Bay.
Waka Kotahi NZTA initially closed State Highway 2 near the plant, between Hunter Brown St and Mahia Ave. The road has now been reopened but is subject to temporary speed restrictions.
Drivers are warned to expect delays and use an alternative route if possible.