Introduction and Aims. To examine Australian opioid substitution treatment (OST) prescribers' perceptions of (i) diversion and/or injection of methadone, buprenorphine, buprenorphine-naloxone by patients; and (ii) effectiveness of current treatment policies in minimising the associated risks. Design and Methods. 1278 authorised OST prescribers, identified by each jurisdiction's health department records, were sent a postal survey in 2007. Reminder letters and additional copies of the survey were sent to non-responders at weeks four and eight following the initial mail-out. Respondents went into a draw to win one of ten $100 book vouchers. Results. Although the response rate was 26% (N=291), participating prescribers served half (49%) of all OST patients in Australia. Prescribers perceived more buprenorphine patients removed supervised doses (7%) and diverted unsupervised doses (20%), compared with methadone patients (1% and 4% respectively) and buprenorphine-naloxone patients (3% and 2% respectively). Prescribers reported significantly more buprenorphine and buprenorphine-naloxone patients injected doses (5% respectively), compared with methadone patients (2%). Non-adherence was identified through patient self-report (51%), and the reports of pharmacists (49%) and other staff (34%). More prescribers were confident in assessing the risk of injection (54%) than diversion (37%). Many prescribers responded 'don't know' to quantitative survey items. Qualitative responses highlighted uncertainties in assessing diversion/injection and whether current responses constituted 'best practice'. Discussion and Conclusions. Australian prescribers perceive most patients adhere with OST, although they may underestimate the levels of diversion. Prescribers' beliefs about patients' behaviours are important and influence decisions to prescribe, medication choice and suitability for unsupervised dosing. The uncertainties in assessing and responding to diversion/injection may be a factor deterring prescribers' participation in OST. �� 2011 Australasian Professional Society on Alcohol and other Drugs.