Despite the traumatic and fatal nature of motor neurone disease (MND) and the caring experiences being described as unrelenting, little is known about risk of psychiatric morbidity and Prolonged Grief Disorder (PGD) for family caregivers. Methods: A cross-sectional survey of caregivers bereaved in 2016-2018 was distributed by the five MND Associations in Australia (2019). Validated tools for PGD (PG-13), anxiety, depression, and family functioning were included. Multinomial logistic regression was used to compare the factors associated with grief. Findings: Overall, 393 valid responses were received, a 31% response rate. The prevalence of ICD-11 PGD was 9.7%; moderate/severe anxiety 12.3%, moderate/severe depression 18.5% and 18.7% indicated poor family functioning. MND caregivers have higher bereavement risk prevalence than the general bereaved population, with 9.6% in the high-risk group (vs 6.4%) and 54% at moderate risk (vs 35%). Being in the PGD group was 8 or 18 times more likely when the respondent had anxiety or depression, respectively. Poor family functioning significantly increased the likelihood of PGD by four times. Other significant predictors of PGD were a recent bereavement (<12 months), being a spouse/partner of the deceased, insufficient support during the disease journey, the deceased being under 60 years of age, and a shorter period of caring (<1.5 years). Conclusion: In a large national population-based sample of bereaved MND caregivers, 63% required bereavement support over and above that provided by family and social networks. This is a neglected yet seriously ill population that calls for better care provision and clinical practice.