This 2001–2013 panel study finds 33% of part-time employees to be underemployed, disproportionately affecting males, immigrants, youth, the blue-collared and new to our study, those on casual contracts. Within the underemployed sample, we report that the gap between preferred and actual hours (working hour tension) also varies by personal and labour characteristics, largely consistent with that found for the determinants of underemployment. The continued growth of part-time employment and casualization in recent years has been more pronounced among males, raising their rates of underemployment as they disproportionately prefer to work more hours vis-à-vis their part-time female peers.