New Zealand dairy giant Fonterra, the world's biggest dairy exporter, said on Wednesday its head of NZ Milk Products, Gary Romano, has resigned with immediate effect.

"Gary has made a significant contribution during his time at Fonterra and we respect his decision," Fonterra Chief Executive Theo Spierings said in a statement, adding that he would take over the running of the operation in the meantime.

Fonterra did not say in the statement why Romano resigned.

The resignation followed the discovery of a bacteria that can cause botulism in some Fonterra products, which were shipped to customers in nine countries.

Fonterra's Shareholders Fund, an investment unit based on the company's dividend stream last traded down 0.1 percent at NZ$6.92 ($5.52).