The co-occurrence of substance use and mental health problems, often referred to as dual diagnosis (DD), is increasingly recognised as commonplace within substance abuse treatment programs. Two-hundred and thirty-four individuals from 9 Australian Salvation Army drug and alcohol rehabilitation programs completed a 3-month post-discharge telephone follow-up. Using a cut-off score from the Psychiatric Subscale of the Addiction Severity Index (5th ed.), 66.7% were classified as likely to have DD and 33.3% as substance use disorder only (SUD). Both groups reported comparable and decreased substance use levels at follow-up, yet DD individuals perceived less improvement in substance use problems. Comparable improvements were reported in the areas of: symptom distress and recovery from symptoms. This was despite greater scope for improvement in individuals with DD. Duration of treatment and access to post treatment services were also assessed. Understanding factors effecting treatment outcomes is imperative for the implementation of effective, evidence based treatment programs.