A microbiologist says the Fonterra contamination scare raises questions about cleaning procedures at milk-processing plants.
Fonterra says the bacteria got into its products through a pipe at a processing plant in Waikato.
Professor of food microbiology at Auckland University of Technology, John Brooks, says the bacterium is extremely rare in raw milk, and it is possible the pipe had dirt or mud on it when it was installed.
Professor Brooks says even then, standard cleaning procedures should have wiped out the bacteria, which brings into question whether those procedures were followed.
Elswhere, commentators say the contamination scare has highlighted the dangers of New Zealand's reliance on the dairy sector.
President of the New Zealand Association of Scientists, Shaun Hendy, says more effective policies are needed to develop other industries, and reduce dairy's influence.
But Waikato University professor, Jacqueline Rowarth, says past efforts to shift the economy away from agriculture have failed.