The Copenhagen Burnout Inventory (CBI) is a public domain questionnaire measuring the degree of physical and psychological fatigue experienced in three sub-dimensions of burnout: personal, work-related, and client-related burnout. This study first examines the reliability and validity of the CBI in measuring burnout in New Zealand secondary school teachers, and then the relationship between burnout and wellbeing among this population. The CBI had acceptable reliability (internal consistency and homogeneity) as well as factorial and criterion-related validity. As expected, burnout was negatively related to wellbeing measures (wellbeing index, school connection, and perceived general health). The findings indicate that this burnout questionnaire is a valid instrument to use with New Zealand secondary teachers, and also highlight the potential impact of burnout on the health and wellbeing of teachers.