Grassland communities worldwide have been extensively modified or lost following broadscale agricultural expansion. In south-eastern Australia few natural grasslands remain, with most now being small, isolated and degraded. Conservation and restoration of grassland communities requires an understanding of the impacts of fragmentation on genetic and demographic processes. Swainsona sericea is a perennial grassland herb with conservation listing in New South Wales, Victoria and South Australia. Reproductive output, progeny fitness and genetic diversity were assessed in nine S. sericea populations occupying fragmented grasslands across the southern tablelands of New South Wales. Unexpectedly, four chromosome classes were observed among the populations (2n = 4x = 32, 2n = 10x = 80, 2n = 14x = 112, 2n = 16x = 128), suggesting a more complex taxonomy than is currently recognised. There was no association between reproductive output and population size or ploidy level whereas population size influenced the number of alleles and percentage of polymorphic loci while ploidy influenced effective alleles and expected heterozygosity. Restricted maximum likelihood analyses of progeny growth indicated that ploidy had a significant influence on height, shoot weight, shoot to root ratio and days to germination. The cytological complexity in S. sericea requires clarification, including delineating the cytological boundaries to enable land managers to include this in their conservation and management plans.