People with a mental health condition have a shorter life expectancy than the general population. This is largely attributable to higher rates of chronic disease and a higher prevalence of modifiable health risk behaviours including tobacco smoking, alcohol consumption, poor nutrition, and physical inactivity. Telephone support services offer a viable option to provide support to reduce these health risk behaviours at a population-level; however, whilst there is some research pertaining to Quitlines, there is limited other research investigating whether telephone services may offer effective support for people with a mental health condition. This protocol describes a randomised controlled trial that aims to evaluate the referral of people with a mental health condition to a population-level telephone coaching service to increase physical activity, healthy eating, or weight management, and increase attempts to do so. A parallel-group randomised controlled trial will be conducted recruiting participants with a mental health condition through community mental health services and advertisement on social media. Participants will be randomly assigned to receive either a health information pack only (control) or a health information pack and a proactive referral to a free, government-funded telephone coaching service, the NSW Get Healthy Coaching and Information Service® (intervention), which offers up to 13 telephone coaching calls with a University Qualified Health Coach to assist with client-identified goals relating to physical activity, healthy eating, weight management, or alcohol reduction. Data will be collected via telephone surveys at baseline and 6 months post-recruitment. Primary outcomes are as follows: (1) minutes of moderate to vigorous physical activity per week, (2) serves of fruit consumed per day, (3) serves of vegetables consumed per day, and (4) a composite measure assessing attempts to change at least one health risk behaviour (any attempts to change physical activity, fruit consumption, vegetable consumption, or other parts of nutrition). Secondary outcomes include weight and body mass index. This study is the first to evaluate the effectiveness of referral to a population-level telephone support service for reducing health risk behaviours relating to physical activity, healthy eating, and weight in people with a mental health condition. Results will inform future policy and practice regarding the delivery of telephone-based behaviour change coaching services and the management of physical health for this population to reduce health inequity and the burden of chronic disease. The Australian New Zealand Clinical Trials Registry ACTRN12620000351910 . Retrospectively registered on 12 March 2020.