Reducing the incidence of blood-borne infections (BBVs) continues to present a considerable public health challenge. Both internationally and in New Zealand, the high risk of transmission of HIV and hepatitis C (HCV) among injecting drug users (IDUs) has led to the introduction of harm reduction measures, such as needle exchange programmes. These measures aim to reduce the potential harm to IDUs and to prevent this group becoming a reservoir for BBVs in the community.This research aimed to analyse the results of the 2009 Needle Exchange Blood Borne Virus Serosurvey in order to estimate the current prevalence of HIV and HCV, and the frequency of behaviours associated with the transmission of BBVs in IDUs in New Zealand who attend needle exchanges. New Zealand's Needle Exchange Programme outlets have taken part in three previous cross-sectional surveys of risk behaviours and prevalence of BBV infections among IDUs. The research also aimed to identify trends over time in seroprevalence and in risk behaviours.The 2009 Needle Exchange Blood Borne Virus Serosurvey is a national survey of needle exchange attendees in New Zealand. Over a two-week period, all IDUs attending the participating 11 needle exchanges were invited to complete an anonymous questionnaire collecting demographic and behaviour data and provide a finger-prick blood sample for serological testing. Replication of previous methodology allowed for comparison of results over time.In total, data from 480 respondents were included in the analysis. The majority of respondents were male (67.3%), of European ethnicity (79.4%) and aged between 35 and 49 years. Average duration of injecting drugs was 17 years. Overall, the seroprevalence of HIV in this population has remained very low (<1%) and there has been a significant decline (17.6%) in the seroprevalence of HCV between 2004 and 2009 (p<0.0001). Prevalence of HCV-antibody was associated with older age, longer duration of injecting, and a history of imprisonment, methadone treatment and tattooing. There has been a significant increase (20%; p<0.001) in the reported use of new needles and syringes for every injection.The decline in HCV seroprevalence in injecting drug users cannot be attributed to any particular differences in demographic characteristics of survey participants or to trends in risk behaviours associated with risk of HCV transmission. The national introduction of the free one-for-one needle and syringe exchange programme in 2004 has had a large impact on growth in distribution at needle exchanges. This reduction in economic barriers to safe injection and subsequent increased access to sterile needles and syringes appears to have had a positive impact on injecting practice and is possibly a factor behind the decline in HCV seroprevalence among New Zealand IDUs who use the needle exchange programme. The 2009 Needle Exchange Blood Borne Virus Serosurvey contributes to the evidence base supporting the effectiveness of needle exchange programmes in reducing injecting risk and limiting the spread of HIV and HCV among IDUs.