Fonterra's claim that its Anchor lightproof bottles protect vitamins in the milk has proven to be false, Consumer NZ says.
The agency tested Anchor trim milk and found the levels of vitamins A and B2 in milk from opaque, clear and semi-opaque bottles were the same.
Consumer NZ chief executive Sue Chetwin told Newstalk ZB it was disappointing that Fonterra made the claims.
"We talked to Anchor about it, they basically said 'yeah you're right, it doesn't protect the vitamins more than others but it does protect taste'.
"I think its disappointing that a dairy giant the size of Fonterra thinks its okay to treat consumers that way," Chetwin said.
"The opaque bottles are generally more expensive than their competitors and certainly more expensive than the house brands."
Chetwin said shoppers who bought the milk because they thought it had higher levels of vitamins in it have been misled.
After Consumer NZ contacted Fonterra to discuss the findings of their testing the dairy company had changed its website to remove the claim, Chetwin said.
A Fonterra spokesman said the intention with the lightproof bottles was "to make sure consumers receive their milk in the best condition it can possibly be in, free from light strike and tasting as it did when it was first bottled."
Fonterra said it couldn't comment on the methodology behind the Consumer NZ research, but said academics had proven milk is damaged by exposure to light.
"When light interacts with these vitamins it can break them down into free-radicals - the biggest driver of protein and fat oxidation in milk.:"
Fonterra said Anchor consumers can taste the difference and that was most important.
In 2013, maths whiz Tristan Pang, then 11, found that milk degraded faster in the Anchor's triple-layer bottle than in the single layer.
In his project, "Triple Layer Milk Bottle - is it effective?", Tristan did three lots of testing on the triple-layer Anchor bottles for taste, light and acidity.
The first two tests received positive results, but Tristan said "something funny" occurred during acidic testing when the milk was left outside: milk in the triple layer bottle degraded faster.
Pang, now 15, is studying towards a Bachelor in Science at the University of Auckland. When he graduates university next year, he'll be 16.
Pang plans to eventually work in the science research field, most likely in quantum physics or medicine.