The ruling was developed by the joint food regulatory body, Food Standards Australia New Zealand (FSANZ), which said research indicated diets in both countries were lacking in iodine.
This could damage the thyroid gland and therefore metabolic processes and the maintenance of body temperature.
Iodine has also been shown to be important for the development of fetal and infant brains.
“Not having enough iodine during pregnancy and early childhood can cause developmental delay and lead to reductions in mental performance,” said the FSANZ’s chief scientist, Dr Paul Brent. “This damage prior to 2-3 years of age is irreversible.”
The ruling, which came into force on October 9 for Australia and September 27 for New Zealand, requires the replacement of regular salt in bread with between 25 and 65mg of iodised salt, but organic bread is exempt.