WELLINGTON -- Another New Zealand milk product has been found to be contaminated after being dispatched for export to China, but New Zealand officials said Monday that none of the products had reached consumers.

The Ministry for Primary Industries (MPI) announced that it had revoked export certificates for four consignments of lactoferrin, made by Westland Milk Products, after they were found to have excessive levels of nitrate.

The consignments came from two affected batches of lactoferrin, a naturally occurring milk protein, made by a Westland factory in Hokitika, on the west coast of the South Island, said an MPI statement.

One batch was exported directly to China by Westland as an ingredient for other dairy products and the second was supplied to the North Island-based Tatua Co-operative Dairy Company, and also exported to China.

Both companies had told the MPI that a small amount of the lactoferrin was used in consumer products, but none of the products had reached consumers.

"MPI's technical experts have looked closely at this issue and believe any food safety risk to Chinese consumers is negligible because the quantities of lactoferrin used in consumer products was very small, meaning the nitrate levels in those products would easily be within acceptable levels," MPI acting director-general Scott Gallacher said in the statement.

"MPI, the Ministry of Foreign Affairs and Trade and the companies concerned are working closely with the Chinese authorities on this issue."

MPI had sent a team to the Hokitika factory to confirm how the problem arose, and to verify that it was limited to just the two batches identified, which appeared to be the case so far.

"The consignments exported to China were accompanied by official export certificates stating that the product complies with New Zealand and China's regulatory requirements. This was based on testing of composited batches undertaken at the time of manufacturing, which showed no issue. We now know that is not the case and certification has been withdrawn," Gallacher said.

Westland Milk Products said all the affected product had been traced and quarantined, although none of it posed a food safety risk.

Chief executive Rod Quin said Westland's two batches of lactoferrin, totalling 390 kg, showed nitrate levels of 610 and 2, 198 parts per million respectively. The New Zealand maximum limit for nitrates is 150 parts per million.

Routine testing prior to export had failed to identify the problem, Quin said in a statement.

"We immediately initiated a process to find and quarantine all of the product and it has been put on hold," Quin said.

Nitrates were a naturally occurring substance found in foods such as leafy green vegetables, and the issue was not the fact that it was present in the lactoferrin powder, but that it was over allowable levels, he said.

"Westland is of the view it is an isolated incident in the lactoferrin plant only, where traces of cleaning products, which contains nitrates, were not adequately flushed from the plant prior to a new run of product," he said.

Earlier this month, China banned some Fonterra products after the company revealed that 38 tonnes of whey protein used by other manufacturers, including makers of infant formula, was contaminated with a bacteria that can cause botulism.