Objectives: This study sought to assess the association between long working hours, psychosocial safety climate (PSC), work engagement and new major depression symptoms emerging over the next 12 months. PSC is the work climate supporting workplace psychological health.

Setting: Australian prospective cohort population data from the states of New South Wales, Western Australia and South Australia.

Participants: At Time 1, there were 3921 respondents in the sample. Self-employed, casual temporary, unclassified, those with working hours <35 (37% of 2850) and participants with major depression symptoms at Time 1 (6.7% of 1782) were removed. The final sample was a population-based cohort of 1084 full-time Australian employees.

Primary and secondary outcome measures: The planned and measured outcomes were new cases of major depression symptoms.

Results: Long working hours were not significantly related to new cases of major depression symptoms; however, when mild cases were removed, the 41–48 and ≥55 long working hour categories were positively related to major depression symptoms. Low PSC was associated with

a threefold increase in risk for new major depression symptoms. PSC was not related to long working hours, and long working hours did not mediate the relationship between PSC and new cases of major depression symptoms. The inverse relationship between PSC and major depression symptoms was stronger for males

than females. Additional analyses identified that work engagement was positively related to long working hours. Long working hours (41–48 and ≥55 hours) mediated

a positive relationship between work engagement and major depression symptoms when mild cases of major depression were removed.

Conclusion: The results suggest that low workplace PSC and potentially long working hours (41–48; ≥55 hours/ week) increase the risk of new major depression symptoms. Furthermore, high work engagement may increase long working hours and subsequent major depression symptoms.