More New Zealand milk products sold to China have been banned after elevated levels of nitrates were found.

The move raises further concerns over quality and testing in the world's largest dairy exporter in the wake of a contamination scare earlier this month.

New Zealand's agricultural regulator said it has revoked export certificates for four China-bound consignments of lactoferrin manufactured by Westland Milk Products after higher than acceptable nitrate levels were found by tests in China.

Two of the four consignments had been shipped to China, but had not reached consumers, New Zealand's Ministry of Primary Industries (MPI) said.

The announcement comes just weeks after Westland's much bigger competitor, Fonterra, said some of its dairy ingredients were contaminated with a botulism-causing bacteria.

This prompted a recall of infant formula products, sports drinks and other products in China, New Zealand and other Asia-Pacific nations.

"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", Scott Gallacher, MPI acting director-general said.

China halted all imports of the product from Westland and asked other New Zealand dairy companies exporting lactoferrin to provide nitrate test reports.

The four consignments were derived from two affected batches of lactoferrin, a naturally occurring protein found in milk, manufactured by Westland at its Hokitika factory on the country's South Island.

Initial investigations pointed to contamination by cleaning products that contain nitrates that were not property flushed from the plant.

Privately owned Westland makes about 120,000 tonnes of dairy product each year, exporting the majority.

Its production pales in comparison with that of Fonterra, which exports 2.5 million tonnes of product.