Fire and Emergency New Zealand has given the "all clear" to the site of an ammonia leak in West Auckland tonight.

Shift manager Carlos Dempsey said the ammonia leak had been isolated and they had confirmed residual gas was dissipating.

He said it took some time to isolate the issue due to the location and technical nature of the leak.

An alert issued advising residents to stay in their homes was now being reversed and locals were able to leave their houses.

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Fire and Emergency responded to the "significant" chemical incident at Westpac Mussels Distributors in Henderson after receiving a 111 call at 6pm this evening.

Residents living south of the factory earlier received alerts to "stay inside, close all windows and doors and await further instructions".

Firefighters in chemical suits at the scene. Photo / Hayden Woodward
Firefighters in chemical suits at the scene. Photo / Hayden Woodward

Five fire trucks attended the scene, as well as a Hazmat unit with personnel in full body protective chemical suits.

One firefighter was later taken to hospital as a "precautionary measure" over fears they may have inhaled ammonia fumes, Dempsey said.

The refrigerant leak appeared to be high up in the building and aerial ladders assisting site technicians were used to isolate the leak and take measurements of the level of ammonia in the area.

Dempsey believed mussel workers were at the site when the leak was reported, but did not know how many people were there.

Ammonia is a refrigerant. It is colourless with a "nasty" smell.

An alert was sent to residents south of the factory, warning them to stay indoors as a precautionary measure due to the wind direction. Photo / Dean Purcell
An alert was sent to residents south of the factory, warning them to stay indoors as a precautionary measure due to the wind direction. Photo / Dean Purcell

Police initially said residents were being evacuated for their safety but later said evacuations were no longer required. It was decided it would be safer and easier for residents to remain inside.

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There were no reports of injuries.

An investigation is expected into the cause of the incident.