Under siege: From left, Gary Romano, Fonterra's head of NZ Milk Products, and Kerry Underhill, the company's brand manager.

Fonterra is in damage control, with its CEO en route to China to meet with companies who use the dairy giant's raw materials.

While Fonterra refused to name the eight companies involved in the investigation, its chief executive is Theo Spierings is returning from Europe via China following news of the contamination.

"It's part of the reason he's going," Fonterra group communications director Kerry Underhill said.

Fonterra has passed on advice to the affected companies and it is up to them and their regulators to manage the risk, Fonterra managing director NZ Milk Products Gary Romano said.

He said Fonterra-labelled products are not among those being investigated, but third-party manufacturers which used its raw materials are.

The presence of bacteria came to light only when Fonterra went to use some of the infected whey product samples as ingredients to make its own products in March this year.

The products underwent more rigorous testing for this, revealing presence of the virulent in July.

"This was surprising for us and our technical staff, we never had any expectation that this would be found in our products," Romano said.

It was the first time such a bacteria had been found in Fonterra products to his knowledge, Romano said.

Fonterra's investigations were voluntary because the amount of bacteria found was at an acceptable level, therefore it passed initial tests, Romano said.

"The testing regimes are based on history and accumulative knowledge.

"The initial component was based on tests required in May 2012."

The trigger for Fonterra's action was that it had found "some" bacteria, he said.

The effects of ingesting the bacteria were still unknown as it depended on the amount of bacteria, the amount consumed and the age of the consumer, Romano said. "The organism is prevalent in soils.

"Small amounts of dust accumulated in the pipework which triggered the contamination."

He said Fonterra had not set about improving its hygiene monitoring processes at its factories as the company was focused on quickly managing the risk to consumers. Romano said Fonterra is otherwise confident in its processes.

Fonterra manufactures 2.5 billion tonnes of product each year.