WELLINGTON -- New Zealand food safety regulators and dairy giant Fonterra on Wednesday defended their "precautionary" approach in issuing an international alert on dairy products wrongly thought to be contaminated with botulism-causing bacteria, while critics branded the crisis a "fiasco."

Opposition leaders demanded stronger regulation after the Ministry for Primary Industries (MPI) revealed that the whey protein concentrate (WPC) at the center of the scare early this month had not been contaminated with a potentially toxic botulism- causing bacterium.

Releasing a report on the tracing and verification of the WPC produced at Fonterra's North Island Hautapu plant in May last year, the MPI said the organism that sparked the alert was actually Clostridium sporogenes, which was incapable of producing botulism toxins.

"There are no known food safety issues associated with Clostridium sporogenes, although at elevated levels certain strains may be associated with food spoilage," said a statement from MPI.

The report said tests in Australia in March this year had detected a species of Clostridium bacteria in a product that was not specific to the potentially toxic Clostridium botulinum, which was linked to botulism.

This was traced back to the whey protein concentrate and further testing in New Zealand "presumptively confirmed" on July 31 this year that it was Clostridium botulinum, said the report.

"When MPI received information from Fonterra on Aug. 2 that it had detected Clostridium botulinum in some of its products, I immediately adopted a precautionary approach to protect consumers both here and overseas," MPI acting director-general Scott Gallacher said in the statement.

"We needed to act on what we knew at that time. The information we had then said there was a food safety risk to consumers and we moved quickly to address it."

At the same time, MPI commissioned 195 further tests using a range of technologies in laboratories in New Zealand and the United States.

Results from the most definitive of these tests arrived over Tuesday night, and were assessed Wednesday, he said.

"All came back negative for Clostridium botulinum," said Gallacher.

The report said all potentially affected product had been accounted for.