CHINESE officials have responded constructively to the discovery of excessive nitrate levels in milk products from Westland Milk Products that were shipped to China, Prime Minister John Key said.

The Ministry for Primary Industries yesterday announced it had revoked export certificates for four consignments of lactoferrin made by Westland Milk after unacceptable levels of nitrates were found in two batches.

The nitrates, reportedly from detergent products not properly flushed away in a plant in Hokitika, were missed in New Zealand testing, only to be picked up by a Chinese customer.

"The Chinese authorities were working very constructively with us over the weekend," Key said at his post-cabinet media conference.

The nitrate find is regarded as a "technical breach" rather than a food safety issue, he said.

"Clearly we're operating in a more sensitive environment so it is particularly unhelpful," Key said in Wellington.

The fact that it wasn't detected in the first instance "again sounds odd".

One batch of the Westland Milk lactoferrin was exported directly to China and a second was supplied to Tatua Cooperative Dairy Co before being shipped to China, the department said.

MPI acting director-general Scott Gallacher said export certificates had originally been issued "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"We now know that is not the case and certification has been withdrawn," he said.

"MPI has been advised by Westland, Tatua and their customers that a small proportion of the lactoferrin was used in consumer products. Almost all of these products are now confirmed as detained in the supply chain."

China has halted all imports of Westland Milk product and asked other New Zealand producers of lactoferrin to provide nitrate test reports.