The life expectancy of adults with mental illness is significantly less than that of the general population, and this is largely due to poor physical health. Behavioural counselling can improve physical health indicators among people with non-communicable disease. This repeated-measures, single-group intervention trial evaluated the effects of a 19-week behavioural counselling programme on metabolic health indicators and physical activity levels of outpatient adults with mental illness. Sixteen participants completed the intervention that comprised individual face-to-face counselling sessions with a registered nurse every 3 weeks, and progress reviews with a medical practitioner every 6 weeks. Assessment included self-report and objective measurement of physical activity, and measures of blood pressure and anthropometry. Statistically-significant changes were demonstrated between baseline and post intervention for participants’ waist circumference (P = 0.035) and waist-to-height ratio (P = 0.037). Non-significant improvements were demonstrated in weight and physical activity. The findings indicated that adults with mental illness can engage in a nurse-led behavioural counselling intervention, with improvements in some metabolic health measures after 19 weeks. It is recommended that behavioural counselling programmes for adults with mental illness be sustained over time and have an ‘open door’ policy to allow for attendance interruptions, such as hospitalization.