Greenpeace is calling on Fonterra to prove the New Zealand dairy industry's use of palm kernel expeller (PKE) is not fuelling rainforest fires in Indonesia.
The substance is a by-product of the palm oil industry and is used as a supplementary animal feed, primarily by the dairy industry.
New Zealand imported a record 1.95 million tonnes of PKE in the year to June.
Greenpeace wants the dairy giant to disclose exactly where it's PKE is coming from.
Fonterra urgently needs to provide credible evidence that its use of PKE is not contributing to the fires, which are caused by the clearing of rain forests for palm plantations, Greenpeace executive director Russel Norman said.
In a letter to Dr Norman, Fonterra's director of social responsibility, Carolyn Mortland, said the co-operative was concerned about the fires in Indonesia and that it was a member of the Round Table for Sustainable Palm Oil (RSPO).
It sources PKE through an Asian agri-business group, Wilmar International, which had a "No Deforestation, No Exploitation" policy, Ms Mortland said.
But, Dr Norman said that was not good enough.
"The RSPO say that they're supplying about 18 percent of global supply from palm products - more generally is RSPO is certified? New Zealand is getting 30 percent of the global supply of palm kernel so there's obviously a big gap there.
"The thing about RSPO is you can get a RSPO certificate even if you're clearing forest and draining peatland, which of course is a key driver of the fires."
Greenpeace will continue to request a meeting with Fonterra head Theo Speirings Russel Norman said.
Fonterra provided Ms Mortland's letter to RNZ but declined interview requests.