Overeducation is a form of labour underutilisation which occurs when the formal education level of a worker exceeds that which is required for the job. In Australia close to 30 per cent of workers are overeducated and are underutilising their skills. Using data from the Negotiating the Life Course survey, this study determines the causes of overeducation in Australia. Four of the key theories that have been used to explain overeducation are tested: human capital, job competition, assignment and the career mobility theories. Tests show that the job competition model best explains the existence of overeducation in the Australian labour market.