Whanganui's Affco Imlay plant has been evacuated for the second time in two weeks because of an ammonia leak.

The plant was evacuated about 3.15pm on Thursday, January 25. It follows a significant ammonia leak on January 11 that left three people, including a pregnant woman, needing hospital treatment after being exposed to the toxic gas.

Fire and Emergency Whanganui station officer Charlie Bilby said this week's incident was "nowhere near as bad" as the previous one.

"It was a lot smaller and the wind direction was in our favour as it was blowing across the [Whanganui] river so the risk to the public was a lot less," Mr Bilby said.

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Four fire trucks attended the incident and fire crews assisted Affco's engineering department to shut down the leak and ensure safety of the staff and public.

A St John spokeswoman said no ambulances were called to Thursday's incident.

WorkSafe said earlier this week it was making inquiries into the January 11 leak but it had not been progressed to an investigation.