Morphological variation in flowers, fruit and leaves of five Australasian species of Avicennia was assessed using multivariate techniques. Groupings based on 184 herbarium specimens gathered throughout the region showed no intermediates or potential hybrids between species, A. alba, A. integra, A. rumphiana (= A. lanata), A. marina (= A. eucalyptifolia; = A. balanophora) and A. officinalis. The most common and systematically troublesome species, A. marina, was considered further in a regional litter fall survey of 25 Australian sites, and in a detailed field study of eight locations within one estuary. In both studies, morphological variation correlated with environmental factors including air temperature, rainfall, intertidal position and upriver location. Major differences were also observed within individuals, as shown in sun and shade leaves. These observations suggested that many attributes, especially of leaves, were influenced by environmental factors, demonstrating their unsuitability in earlier systematic treatments. Other attributes reflecting putative subspecific genetic variation, included bark character, extent of pubescence on calyx lobes, and stigma position in relation to anthers. These characterise three groupings of A. marina sites, related to major biogeographic zones within this region: southeastern Australia and New Zealand; northern and north-eastern Australia and southern Papua New Guinea; and south-western Australia.