New Zealand government ministers say they're working round the clock to limit the international impact of dairy giant Fonterra's whey contamination crisis.

Fonterra has launched an international investigation to get to the bottom of the "human error" that caused a whey protein product to be contaminated with bacteria that could lead to botulism.

The government meanwhile is in damage control on the international front.

"We have made all efforts to inform regulatory authorities in known affected countries and have provided full information to help those authorities trace and recall products and advise consumers," the trade minister, Tim Groser, and the primary industries minister, Nathan Guy, said in a joint statement on Thursday.

"New Zealand is also providing updated information to all other unaffected markets to maintain the transparent and frank approach adopted since the start of this situation."

The government's top priority is still consumer safety but the statement indicates ministers are now starting to focus on repairing the damage to New Zealand's reputation as a safe food supplier.

Groser says New Zealand has ceased issuing export certificates for any potentially risky products.

Guy said only a very limited range of dairy export products were at risk of contamination.

"Most exports are unaffected, including our major exports of whole milk powder and skim milk powder, butter and cheese," he said.

"Our trading partners can continue to deal in these products with full confidence in their quality and safety."

Fonterra said on Wednesday all potentially affected products were off the shelves. "All the stocks have been contained, everything is out of the market," chief executive, Theo Spierings, said. "It's in warehouses and there is little or no more risk for consumers."

The prime minister, John Key, said that when the dust had settled there would be a wide-ranging government inquiry to find out what went wrong, and why.