Investment regulator the Financial Markets Authority says it is concerned about the time it took for Fonterra to disclose the potential contamination of its whey protein to the share market.
Fonterra had test results confirmed last Wednesday indicating the potential presence of a strain of clostridium in a sample, which can cause botulism.
The company has said it immediately contacted its customers and the authorities. The New Zealand Government was informed on Friday afternoon.
But it took until Monday morning for Fonterra to release an announcement to the New Zealand stock exchange where units in Fonterra Shareholders Fund are listed.
Units in the fund slumped immediately to $6.50 from its Friday closing price of $7.12. They have since recovered and last traded at $7.
A spokesman for the FMA said it would be raising the matter directly with Fonterra.
"FMA is concerned about the time it took Fonterra to disclose the potential contamination of whey protein to the market. We will be raising the matter directly with Fonterra."
A spokeswoman for stock exchange operator the NZX said it did not comment on individual issuers.