Research suggests that frequent empathic engagement with others in distress places helpers in registered professional roles (e.g. medical practitioners, psychologists) at risk of functional impairment related to symptoms of psychological distress, including the delivery of sub-optimal care to patients. Preliminary research suggests that telephone crisis support workers may also be impacted in a similar way. This repeated measures study is the first known research to examine telephone crisis support workers' functional impairment related to symptoms of psychological distress before and after speaking with callers in crisis. A representative sample of telephone crisis support workers from Lifeline Australia participated by completing three surveys: 1) directly before; 2) directly after; and 3) one week after completing a shift on the national crisis line. Surveys included standardised measures of functional impairment, psychological distress, lived experience of mental health issues and suicide, motivations for volunteering, coping strategies and help-seeking. Categorical items were used to assess personal and shift-related factors. Repeated measures analyses of variance were used to identify changes in symptoms of psychological distress and impairment across time points. Structural equation modelling was used to test relationships within a hypothesised model of impairment. A significant proportion of participants reported functional impairment related to symptoms of psychological distress. Significant differences in functional impairment and symptoms of psychological distress were detected, and were associated with different mechanisms, across time points. An important outcome of this study is empirically-supported models which explain how telephone crisis support workers come to experience functional impairment in relation to their TCS role, as well as other work/study, home/family and social/leisure activities. Results warrant the deliberate development and/or modification of existing service strategies to optimise telephone crisis support workers' psychological wellbeing and functioning, including by structuring the work environment and emphasising certain messages during training and supervision.