We investigated the role of normative support, behavioural automaticity, and action control in predicting dental flossing behaviour. Between May and October 2015, 629 Australian young adults completed a questionnaire assessing constructs of normative support and automaticity, and a 2-week follow-up of dental flossing behaviour and action control, resulting in n = 241 persons for longitudinal analysis. Findings supported the hypotheses that the effect of normative support on behaviour would be mediated via automaticity, and the effect of automaticity would be moderated by action control. Current results extend previous research to elucidate the mechanisms that help to understand predictors of oral hygiene behaviours and contribute to the cumulative evidence concerning self-regulatory and automatic components of health behaviour.