This dissertation adopts an organisational behavioural (OB) and management approach to explore the effects of boundary flexibility on role demands and personal wellbeing among New Zealand (NZ) midwives. The paper reviews how research on fatigue has largely cited workload as a cause—reinforcing consequences of role flexibility and transitioning between family and work domains. The study aims to investigate the mediating role of ‘family flexibility ability’ on the workload and work fatigue relationship of Lead Maternity Carer (LMC) midwives. An LMC is responsible for organising and developing maternity care and birth plans. Family flexibility ability is defined as the ability of workers to leave the roles of the family domain for the roles of the work domain (Edwards & Rothbard, 1999; Kossek et al., 2006). The causes of work fatigue are numerous and range from time spent awake to family and work domain responsibilities (Caldwell et al., 2019; Dorrian et al., 2011). In the NZ maternity health system, LMC midwives are contracted by the Ministry of Health (MoH) to provide maternity care for women under a continuity of care (COC) model. COC case-loading model indicate that women register with one LMC for their entire maternity care experience (Ministry of Health, 2019). LMC midwives are self-employed therefore, their work domain roles include diverse components such as 24/7 on-call clinical, psychological, and business management roles. LMC midwives are often females who further need to juggle between the responsibilities of their family and work domain roles. Study data (N = 301) was drawn from the responses of LMC midwives in Phase One of the 2019 New Zealand Midwifery Work and Wellbeing (NZ MidWoW) study. The boundary theory, job demands-resources (JD-R) model, and motivational control theory provide the theoretical framework to explore this data. Consistent with the literature, the study finds that quantity of workload is positively correlated to physical, mental, and emotional work fatigue. Using PROCESS analysis, the research confirms iv the mediating role of ‘family flexibility ability’ in the relationship between quantitative workload and work fatigue in this study’s population. Quantitative workload refers to an excessive amount of work that workers cannot complete within the prescribed period (Van Sell et al., 1981). This dissertation provides new knowledge regarding the relationship between workload and work fatigue among LMC midwives that requires them to regularly address work concerns while in their family domain. It is argued that the mediated pathway approach sheds light on supporting LMC midwives to be more sustainable in their vital role as healthcare providers. Both theoretical and practical implications of findings are discussed.