The New Zealand Food Safety Authority moved to reassure consumers after research found 58% of fortified food products tested contained levels of nutrients different from those declared on their packaging.
Environmental Science and Research, which carried out the study for NZFSA, found 15% of the 260 products probed contained lower levels of nutrients than disclosed on labels, while 42% contained more.
The New Zealand Herald said Dr Barbara Thomson, who conducted the work over three years, had found "glaring differences" in levels of vitamin C, D and A, folate, calcium, iron and selenium found in products such as bread, cereal, fruit drinks and baby food.
According to the newspaper, Thomson said: "The labels don't necessarily reflect what's in the food. Being over the label claim can be just as bad. The work we've done shows that the products are up to five times over."
But variations in nutrient claims were not a food safety concern and were expected as the level of nutrient declared on the nutrition panel was based on an average, said NZFSA assistant director (joint food standards) Jenny Reid.
"A lot of factors will influence the levels in any single serving of food, including analytical methods, storage conditions and the fact that manufacturers need to cater for possible degradation of the nutrient during its shelf life," she said.
Many nutrients were not stable and levels could be affected by the production process, the shelf life of the product and the conditions the product is stored under, she added. To compensate for any degrading over time some manufacturers added more of some nutrients.
Although a large number of the label claims on the foods sampled in the surveys did not meet the actual measured levels there was no cause for concern, insisted Reid, who said: "The permitted claimable levels of the various nutrients have huge inbuilt safety margins."