Objective  Vitamin D deficiency is recognized as a global public health problem, but the population‐based prevalence of deficiency and its determinants in Australian adults is not known. This study evaluated the vitamin D status of Australian adults aged ≥25 years and risk factors associated with vitamin D deficiency in this population. Design and Patients  We studied a national sample of 11 247 Australian adults enrolled in the 1999/2000 Australian Diabetes, Obesity and Lifestyle (AusDiab) study drawn from 42 randomly selected districts throughout Australia. Measurements  Serum concentrations of 25‐hydroxyvitamin D 25(OH)D were measured by immunoassay. Vitamin D deficiency was defined as a concentration <50 nmol/l. Information on demographic and lifestyle factors was derived from interview‐administered questionnaires. Results  The mean serum 25(OH)D concentration was 63 nmol/l (95% CI: 59–67 nmol/l). Only 4% of the population had a level <25 nmol/l, but the prevalence of vitamin D deficiency (<50 nmol/l) was 31% (22% men; 39% women); 73% had levels <75 nmol/l. The prevalence of vitamin D deficiency increased significantly with age, was greater in women, in those of non‐Europid origin, in the obese and those who were physically inactive and with a higher level of education. Deficiency was also more common during winter and in people residing in southern Australia (latitude >35°S); 42% of women and 27% of men were deficient during summer–autumn, which increased to 58% and 35%, respectively, during winter–spring. Conclusion  Vitamin D deficiency is common in Australia affecting nearly one‐third of adults aged ≥25 years. This indicates that strategies are needed at the population level to improve vitamin D status of Australians.