Three estimates of the number of injecting drug users residing within the Eastern Sydney Area Health Service (ESAHS) were obtained by applying the capture-recapture technique to data from three reliable sources, specifically, Methadone Services, the Bourke Street Drug Advisory Service and the Albion Street Clinic. The obtained estimates were 3449, 3046 and 1103 respectively. Statewide methadone statistics were also used to obtain an estimate of 15,082 injecting drug users in New South Wales. This estimate was close to the linearly projected estimates (14,274 and 14,416) from two previous studies. Estimates based upon the methadone statistics indicate that approximately 23% of injecting drug users in New South Wales reside within the ESAHS. A sensitivity analysis was performed to assess the effects on the estimates of the likely violation of the closed population assumption.