Introduction: With recent policy changes around medicinal cannabis in Australia, there is concern about the influence of changing norms of cannabis use on adults who are actively parenting. Methods: This repeated cross-sectional population study used National and Drug Strategy and Household Surveys to estimate the changes and correlates of cannabis-related attitudes (support of legalisation, approve of regular use, would try or use if legal) among Australian parents from 2016 to 2019. Results: The estimated proportion of parents who supported legalisation and approved regular cannabis use increased significantly. Parents who would try cannabis if it was legal grew from 5.9% (95 %CI: 5.2, 6.7) to 8.1% (95 %CI: 7.2, 9.0). Parents who said they would use cannabis more often increased from 1.6 (95 % CI: 1.2, 1.9) to 2.9 (95 %CI: 2.4, 3.4), an 81% jump in the three years. The strongest associations were observed between a very high level of psychological distress and regular smoking and drinking. For example, people with a very high level of psychological distress were 2.16 times (95 %CI: 1.42, 3.28) and 2.48 times (95 %CI: 1.61, 3.83) more likely to approve legalisation and regular cannabis use, respectively. Daily drinking was associated with higher odds of trying cannabis (OR = 1.66; CI: 1.25–2.20). Discussion and conclusion: The proportion of parents who would try or use cannabis more often represents a sizeable pool of potential new and frequent users. This highlights the need for education and intervention strategies for parents who use cannabis and care for young children. The associations between mental health and substance use suggest that more research is needed to understand the impact of legalisation on vulnerable groups.