Obesity and metabolic syndrome are disproportionately prevalent among people with mental disorders, particularly those subject to long periods of inpatient care where sedentary lifestyle and medication side effects can be compounding problems. This study reports on a cross-sectional survey of 28 patients under forensic mental health care in a high secure inpatient service in Queensland, Australia. The study comprises a file review, structured questionnaire and an interview, to evaluate the physical health and activity, as well as possible causes of low physical activity of these patients. Rates of overweight (50.0%), obesity (46.6%), and metabolic syndrome (39.3%) were very high within the study population. Mean waist circumference measurements were also high for both females (110.8 cm) and males (109.0 cm). Around one half of the study population (46.4%) did not meet the World Health Organization’s recommended levels of physical activity. Self-reported low levels of physical activity were significantly associated with the presence of comorbid medical illness (p = 0.007). Further research investigating the factors contributing to poor physical health and low activity levels within this population will support attempts to understand and address the physical health needs of this population.