Objectives Underemployment occurs when workers are available for more hours of work than offered. It is a serious problem in many Organisation for Economic Co-operation and Development (OECD) countries, and particularly in Australia, where it affects about 8% of the employed population. This paper seeks to answer the question: does an increase in underemployment have an influence on mental health?

Methods The current paper uses data from an Australian cohort of working people (2001–2013) to investigate both within-person and between-person differences in mental health associated with being underemployed compared with being fully employed. The main exposure was underemployment (not underemployed, underemployed 1–5, 6–10, 11–20 and over 21 hours), and the outcome was the five-item Mental Health Inventory.

Results Results suggest that stepwise declines in mental health are associated with an increasing number of hours underemployed. Results were stronger in the random-effects (11–20 hours =−1.53, 95% CI −2.03 to −1.03, p<0.001; 21 hours and over −2.24, 95% CI −3.06 to −1.43, p<0.001) than fixed-effects models (11–20 hours =−1.11, 95% CI −1.63 to −0.58, p<0.001; 21 hours and over −1.19, 95% CI −2.06 to −0.32, p=0.008). This likely reflects the fact that certain workers were more likely to suffer the negative effects of underemployment than others (eg, women, younger workers, workers in lower-skilled jobs and who were casually employed).

Conclusions We suggest underemployment to be a target of future workplace prevention strategies.