Providing information about substances injected can reduce the negative impact of illicit drug consumption and support people who inject drugs to make informed decisions. In Australia, information about drugs injected relies largely on periodic self-report surveys. For the first time, the analysis of the residual content of used injecting equipment was conducted in a supervised injecting facility (SIF) located in Sydney, Australia. The aim was to gain a better understanding of the substances injected by clients through: (1) chemical analyses of the content of used syringes; (2) comparison of these results with clients' self-reported drug use; and (3) assessing the usefulness of analysing other injecting equipment to detect substances used. During one week in February 2019, syringes and other injecting equipment were collected at the Sydney SIF. Their residual content was analysed by gas-chromatography/mass-spectrometry. Heroin was the most commonly detected substance (present in 51% of syringes), followed by methamphetamine (22%) and oxycodone (10%). In addition to the main psychoactive substance, cutting agents reported in the literature were also detected in used syringes. The main psychoactive substance identified by laboratory analysis reliably corresponded with users' self-reported drug type. Analytical confirmation of substances injected allows for the provision of better targeted harm reduction messaging based on timely and objective data. The approach used is amenable to clients and feasible in the Australian SIF context. Upscaling and wider implementation could be done through Needle and Syringe Programs, and would support the early detection of harmful substances entering drug markets and better inform harm reduction strategies.