Using 2001-17 panel data, this paper investigates the determinants of underemployment among females in part-time employment. This is a matter of policy relevance given the high and growing rate of part-time employment in Australia and the preponderance of females in this work role. Statistical analysis shows a significant and persistent presence of such underemployment, with further bifurcations based on age, qualifications and the presence of children at home. Econometric results suggest that the young, tertiary educated and those with no children at home are likelier to be underemployed. The underemployed do however, surprisingly, enjoy a 'compensatory effect' with respect to being happier than the non-underemployed with the nature of their work tasks and work-life balance. In all cases, those part-timers who are also casually employed are also more likely to be underemployed.