BACKGROUND: Validation of instruments used to measure physical activity patterns is essential when attempting to assess the effectiveness of physical activity interventions. OBJECTIVES: To assess the validity of two self-report physical activity questionnaires on a representative sample of New Zealand adults. METHODS: 70 adults aged 18-65 years from around Christchurch, New Zealand were required to wear an ActiGraph GT1M accelerometer during all waking hours for 7 consecutive days. Immediately following the 7 day accelerometer period participants were required to complete the long forms of both the New Zealand Physical Activity Questionnaire (NZPAQ-LF) and the International Physical Activity Questionnaire (IPAQ-LF). RESULTS: Both the NZPAQ-LF and the IPAQ-LF questionnaires showed small to moderate correlations with ActiGraph data for time spent in moderate-intensity physical activity (r=0.19-0.30) and total physical activity (sum of moderate and vigorous-intensity physical activity, r=0.30-0.32). In comparison with the ActiGraph data, both self-report questionnaires tended to overestimate activity levels by approximately 165%. Total physical activity levels gathered from both questionnaires were strongly correlated with each other (r=0.79) and showed good levels of agreement in the Bland-Altman plots. CONCLUSIONS: The long forms of the NZPAQ and IPAQ were found to have acceptable validity when detecting participants' ability to meet activity guidelines based on exercise duration, but a significant amount of overestimation was evident. This presents a need for both instruments to be further developed and tested in order to increase validity.