In 2008, the company was caught up in a tainted milk scandal that left six Chinese children dead and sickened nearly 300,000 others. Fonterra, New Zealand's biggest exporter, sells 371,000 tons of infant formula each year to China.

In January, Fonterra products were found to have small levels of dicyandiamide, used in some fertilizers.

Romano said the Clostridium botulinum contamination was a complete surprise.

"We have never had any expectation that this would be found in our products," Romano said, adding that it is very rare for Clostridium botulinum to be found in dairy products.

Fresh dairy products like milk, yoghurt, cheese, spreads and UHT milk products are not affected.

New Zealand Ministry for Primary Industries said in a statement Saturday that it has advised overseas authorities that some products of dairy giant Fonterra may have been contaminated with a potentially deadly toxin.

The ministry said it is working closely with Fonterra on the food safety issue with a range of products manufactured from whey protein concentrate produced at a single New Zealand manufacturing site in May 2012, adding there have been no reports of illness linked to the consumption of the affected product.

The ministry said it has contacted the appropriate regulatory authorities overseas, and is focusing on ensuring there are no contaminated products on the New Zealand market.

"At present, we are continuing to verify information provided to us, and we will update further if any products are identified. Products on the market will be recalled if they are found to contain the contaminated protein," said MPI Acting Director General Scott Gallacher on Saturday.

"We are also working with Fonterra to establish what has happened, how it happened, and what can be done to ensure it does not happen again," Gallacher said.

Botulism is a rare but serious illness caused by a bacterium which occurs in soil. It produces a toxin that affects your nerves. Foodborne botulism comes from eating foods contaminated with the toxin. Intoxication botulism can result when food processing failure and/or temperature abuse allows germination of spores and proliferation of vegetative cells.

Initial symptoms include nausea, vomiting, and diarrhoea. Neurological symptoms follow, beginning with cranial nerve areas including eye, throat and mouth, and then travelling down the body paralysing motor nerves. Lack of muscle co-ordination, fatigue and respiratory impairment are characteristic. Constipation may develop after onset of neurological symptoms; abdominal pain may be present throughout.

Most cases (up to 80 percent) require hospitalization for a 4-5 week period. All botulinum interfere with neurotransmitters, a temporary condition eventually restored by motor endplate regeneration. Effects are not usually long term.