Using repeated, cross-sectional behavioural surveillance data from Australia, we assessed trends in relationship agreements and casual sex among HIV-negative and untested gay and bisexual men who had regular partners during 2013-2018. We conducted three analyses: (i) trends in relationship agreements and casual sex over time; (ii) bivariate comparisons of PrEP users and non-PrEP-users to identify factors associated with PrEP use; and (iii) multivariate logistic regression to identify factors independently associated with PrEP use. The analysis of trends over time included 21,593 men, from which a sub-sample (n = 3764) was used to compare PrEP users and non-PrEP-users. We found a large increase in agreements that allowed condomless sex with casual partners, particularly by PrEP users in relationships (nearly 40% of whom had such an agreement). A further 34% of PrEP users reported having casual condomless sex without an agreement that permitted that behaviour, while 13% of non-PrEP-users also reported condomless sex with casual partners without an agreement. PrEP use was independently associated with having agreements permitting condomless sex with casual partners, recent condomless sex with casual partners, having greater numbers of male partners, recent post-exposure prophylaxis use, having an HIV-positive regular male partner, and recent condomless sex with regular male partners. Our findings show a shift away from relationship agreements in which condomless sex was only sanctioned between regular partners.