Objectives: To examine population iodine status in Tasmania after mandatory iodine fortification of bread and assess the magnitude of difference compared with results from a period of voluntary iodine fortification.

Design and setting: A cross-sectional urinary iodine survey of schoolchildren from classes that included fourth-grade students was conducted in Tasmania in 2011. Results were compared with surveys conducted before fortification and during a period of voluntary fortification.

Participants: Three hundred and twenty students aged 8–13 years from 37 participating school classes.

Main outcome measures: Median urinary iodine concentration (UIC) and proportion of UIC results < 50 µg/L.

Results: Median UIC in 2011 was 129 µg/L, and 3.4% of samples had a UIC under 50 µg/L. This was significantly higher than during the period of voluntary fortification (129 µg/L v 108 µg/L) (P < 0.001), which was significantly higher than before fortification (108 µg/L v 73 µg/L) P < 0.001). There was a reduction in the proportion of samples with UIC under 50 µg/L after mandatory fortification (3.4%) compared with results from the period of voluntary fortification (9.6%) (P = 0.01), which was a further reduction compared with results from the prefortification period (17.7%) (P < 0.001).

Conclusions: Iodine status in Tasmania can now be considered optimal. Mandatory iodine fortification has achieved significantly greater improvements in population iodine status compared with voluntary fortification. However, surveys of schoolchildren cannot be generalised to pregnant and breastfeeding women, who have higher iodine requirements. Measurement of iodine status in population surveys is warranted for ongoing monitoring and to justify the appropriate level of fortification of the food supply into the future.