BACKGROUND: Marginal deterrence refers to deterrence of a more harmful act because its expected sanction exceeds that for a less harmful act. Legitimacy of the law predicts that laws perceived as fair will generate compliance and laws perceived as unfair will generate defiance. The introduction of the Cannabis Infringement Notice (CIN) scheme in Western Australia provided an opportunity to test these theories by assessing whether perceptions of certainty, severity and fairness of punishments dictated by the CIN scheme would affect how regular cannabis users intended to obtain cannabis after legislative change. METHODS: 100 Perth residents (mean age 32.2 years; two thirds male) who reported using cannabis at least once a week were given semi-structured interviews before the CIN scheme came into effect. RESULTS: There was limited opportunity for the CIN scheme to effect marginal deterrence as most of the sample were already purchasing or growing within the lower penalty thresholds. Yet of the minority who were purchasing and growing outside of the CIN scheme, a significant proportion reported intending to change their behaviour to fit within the scheme, including the only purchaser of more than 30 grams and 6 of 14 non-hydroponic growers of 3 or more plants. Perceived certainty, severity and fairness of penalties were not as important in determining purchasing and growing behaviour as factors such as ‘meeting needs’, ‘cost’ and ‘preference’. CONCLUSION: The results suggest that under the CIN scheme, marginal deterrence and legitimacy will likely play only a small or selective role in decisions about obtaining cannabis, although in some areas such as the numbers of non-hydroponic plants marginal deterrence may be evident.