Reliability of online self-report versions of the Australian Treatment Outcome Profile (ATOP) and Alcohol, Smoking, and Substance Involvement Screening Test (ASSIST) among people in residential treatment for substance use problems

•Reliability for online self-report measures was moderate to high for most substance types when measured by the ATOP in people seeking treatment for substance use problems.•An underreporting of lifetime substance use during online self-report of the ASSIST led to poor reliability outcomes for nearly all substance types, indicating the need for researcher and clinician support for these ASSIST items (e.g., through pre-recorded instructions).•Reliability for the ASSIST past 3-month substance use and related harms was moderate to high for all substance types.•Reliability of illicit quantity estimates have previously been avoided for the ATOP, however standardising quantities and calculating the average quantity per day was a reliable estimate for most substance types.•Researchers planning to use online self-report measures of the ASSIST and ATOP among treatment-seeking populations should consider ways to address limitations in reliability for certain frequently used substance types (e.g., alcohol and cannabis) during the study design phase.