Context Despite widespread interest in platypus (Ornithorhynchus anatinus) conservation, it is unclear how their fundamental morphometric and demographic characteristics differ over a range of scales. This hampers impact assessments and understanding of platypus ecology. Although the ulcerative fungal disease mucormycosis has infected platypuses in Tasmania for three decades, its population level impacts and conservation significance remain unknown. Aims This study examined morphometric and demographic patterns in Tasmanian platypuses to provide a basis for investigating impacts of mucormycosis or other anthropogenic disturbances. It also sought to identify important spatial scales of natural variability and the magnitude of seasonal variation in platypus body size, condition and population structure. The hypothesis of higher mucormycosis prevalence and mortality in adult males was also investigated. Methods Extensive live-trapping surveys were conducted from January 2008 to July 2009 in 75 streams and 18 river catchments across Tasmania including King Island. The sex, age, body size, tail volume index, health and moult condition of 195 individuals were assessed, and population age and sex structures characterised. Sampling focussed on assessing variability within and between river catchments and compared populations in river catchments with contrasting disease status. Key results Differences in platypus morphometrics within and between catchments and seasonal moulting patterns were detected. Adult males had higher fat stores than adult females, especially during winter. This study also provided the first evidence of population level consequences of disease in platypuses. The demographic group most commonly affected by mucormycosis was confirmed to be adult males. Differences in male age structure among catchments of varying disease status were consistent with the hypothesis of higher adult male mortality rates and turnover in currently affected catchments. Conclusions More than 25 years after mucormycosis was first detected in Tasmanian platypuses, the disease continues to play a low-level, ongoing role in affected populations. Implications The present study provides the first systematic multi-scale spatial investigation of platypus mucormycosis, which contributes to unravelling the epidemiology of the disease and detecting its impacts. By identifying the magnitude and important scales of morphometric and demographic differences in Tasmanian platypuses the study also assists researchers choose comparable demographic groups and spatial scales for meaningful comparisons in future impact studies.