A food safety expert says "The Collective" yoghurt company, Epicurean Dairy, was lucky listeria didn't get into its yoghurts and onto supermarket shelves.
The company and two directors have been fined almost $500,000 for covering up positive tests for the bacteria over years.
Each time a positive test was returned, the Ministry for Primary Industries (MPI) should have been alerted and production stopped.
Over four-and-a-half years, the company's west Auckland factory tested positive for listeria 235 times.
However, it didn't get from the floor where it was detected, into food. The company's lawyers claimed in court the risk to the public was only theoretical and not real.
But Massey University microbiologist and food safety professor Steve Flint said that was pure luck.
"The chance of getting listeria into product in that environment is pretty high. Even though it's on the floor, there's aerosols from washing the floors and so on that let the organism spread quite readily. It is a very serious issue," Dr Flint said.
Although the risk of being ill due to listeria was small, for people with compromised immune systems, the consequences could be fatal, he said.
"It crosses the brain-blood barrier which is kind of unusual so it can get into the brain and cause meningitis. It crosses the placenta and can cause ... stillbirth and so on. So it's pretty serious in terms of how it reacts within the human body, more so than many other food-borne pathogens like salmonella which simply just makes you sick."
Listeria is common in water and soil, and a stringent regulation system for food manufacturers is overseen by the MPI.
Its compliance director, Gary Orr, said a tip off from an anonymous source led to an investigation into Epicurean Dairy.
The company's general manager, Angus Allan, claimed he had no idea about the positive results and that the architect of the cover up - operations manager Ilya Pyzhanhov - kept him in the dark completely.
Mr Allan is still employed at the company but in a different role.
In a statement the company apologised, saying it had redone the floor and wanted to reassure people that food safety was a top priority.
It said the person responsible left the business three years ago and in the meantime it had made changes and had the highest possible rating for its quality systems.