Firefighters have been working to contain a poisonous gas leak at a meatworks near Ngaruawahia this morning, the second at the company in less than a fortnight.

Fire Service communications spokesman Jaron Phillips said six crews - about 35 firefighters - were called to the Ammonia leak at the Horotiu Affco meat processing plant about 10.15am.

The hazardous materials unit also responded.

The leak was discovered in the engine plant room but had been isolated and the Fire Service was now ventilating the site, he said.

"It's actually just in the last few moments been isolated.

"It's just for the risk [that we attend]. Our guys have entered the site wearing breathing apparatus...you can only enter an area like that wearing protective equipment."

The Fire Service also attended an ammonia leak at the company in the early hours of June 30, he said.

The company and the Fire Service would work together to determine the cause of the latest leak.

"This is the second type of incident like this. It's at the same site," Mr Phillips said.

The leak in June caused a full site evacuation at the meatworks.