"I get the smell coming through here all the time," said Mr Walsh, who was worried about the long-term health effects from smelling the pesticides, which included 1080 poison and cyanide.
He had previously seen people in white hazmat suits outside the ACP building, he said.
While he was not often affected himself, some of the other workers complained of headaches and nausea when the smell was about.
Another builder, Leon Casson, said he could feel it on his face.
"My face is itching," he said.
Mr Casson said he could feel the smell clinging in the back of his throat.
Mr Walsh said it had been going on for year. He had never complained to the company but he had written a letter to the Chronicle once.
The principle substances dealt with at ACP were 1080 and brodifacoum, an anticoagulant poison, neither of which was volatile, Mr O'Connor said.
"Any odour from the site will not contain these compounds.
"There is a smaller quantity of cyanide paste dealt with at ACP.
"This is most unlikely to form a gas, and there are controls in place to protect workers in the factory," he said.
"If that was causing problems offsite, there would be huge problems onsite."
Mr O'Connor said the process of making baits involved flavoured lures and the baking of cereal. "I cannot rule out the possibility that this may somehow contribute to the reported odour."
This was still not harmful to anyone who could smell it though, he said.
"If people have ongoing concerns, they are very welcome to contact us directly."
People with health-related inquiries can contact Public Health, or Health and Safety at the Ministry of Business, Innovation and Employment.
Animal Control Products management said they were "forbidden to speak to the media".