An understanding of animal population dynamics relies on identifying life-history attributes associated with population growth and determining how these are affected by environmental variables. We analysed platypus population processes over a 10-year period through mark–recapture studies conducted in three spatially independent stream systems located in the suburbs of Melbourne, Australia. The three populations were collectively characterised by a slightly male-biased adult sex ratio (1.15 : 1) and relatively low reproductive success (<0.5 juvenile captured annually per adult female). An estimated 16% of core residents disappeared annually and 18% of marked juveniles were recaptured as adults. However, some demographic parameters (reproductive success, frequency of non-core adult captures) varied significantly among populations. Estimates of annual core population size in the three systems varied asynchronously, with different trajectories in population size potentially reflecting habitat differences (amount of urban development, reliability of surface flow) as well as variation in spatial isolation and catchment history (implementation of stream rehabilitation programs, occurrence of severe floods). Across all three populations, significant variability in annual reproductive success was explained by linear relationships with the amount of rainfall recorded in the five months before breeding (positive) and after juveniles emerge from nesting burrows (negative).