Officials investigating the recent outbreak of a severe gastro bug believed it was better to work with growers and retailers, rather than panic the public.
Documents released under the Official Information Act show the Ministry for Primary Industries (MPI) was mindful of balancing the risk of further illness against the risk of panicking the public and a loss of trust in the food supply chain.
An email from MPI, dated 1 October, said it considered "there will be greater ongoing positive effect and influence, with lesser risk of negative results, by managing the food safety hazard at the most likely source, ie: with industry".
MPI had been visiting farms and retailers to try and pinpoint the source of the bug.
'Sufficient to focus the investigation'
A report by the Institute of Environmental Science and Research (ESR) on 6 October named carrots and Pam's bagged lettuce from Foodstuffs' supermarkets as the most likely sources of the bacteria.
MPI refused to tell the public which vegetables were suspected of being the source and said the ESR's conclusions were "not definitive".
However, a briefing to the Minister of Primary Industries, Nathan Guy, and the Minister for Food Safety, Jo Goodhew, on 10 October stated the ESR report "is sufficient to focus the investigation of possible contamination of both lettuce and carrots at some point between growth and consumption".
Canterbury's medical officer of health, Alistair Humphrey, decided to name the two vegetables that week, despite the Ministry asking him not to.
That weekend, MPI officials met with supermarkets, retailers and key industry bodies to discuss the issue and try and track their supply chains in a bid to pinpoint the source.
Foodstuffs, which owns New World and Pak'n'Save, later confirmed some of its produce may have been contaminated. It is expecting test results next week.
Washing 'will not afford protection'
The documents also showed MPI believed the best it could do was inform the public to wash all fruit and vegetables as a precaution.
But, in an email dated 1 October, MPI said it was likely that the suspected vegetables were contaminated with the bacteria internally, rather than just on the surface: "Meaning that washing of the produce by consumers will not afford protection from illness." This information was not passed on to consumers.
A briefing paper dated 9 October said MPI had also informed overseas markets that had bought New Zealand lettuces and carrots of the outbreak, "noting that these products have not, to date, been confirmed as a source of the outbreak".
The names of companies and certain parts of the reports that were released to Radio New Zealand News have been edited out, with MPI citing commercial reasons.
Since 1 September, 170 people have contracted Yersinia pseudotuberculosis and 72 people needed hospital treatment. There are a further 59 suspected but not confirmed cases of infection.