AbstractAn assessment of the performance of a wetland dominated by opportunistic weeds in removing nutrients from a secondary sewage effluent was carried out at Thredbo in Kosciusko National Park. Water quality sampling of the inflow and outflow showed a reduction in turbidity, pH and conductivity with passage through the wetland. Dissolved oxygen levels also decreased. In summer 1982, the phosphorus toad of 1.8 kg P day‐1was reduced by 44% as wastewater moved through the wetland, and the nitrogen load of 6.4 kg N day‐1by 65%. Winter retentions were 10% and 14% of inflowing loads, respectively. In 1983 there was a net release of phosphorus and reduced retention of nitrogen. This was regarded as being a result of physical disturbance of the wetland. Vegetation downstream of the effluent inflow was dominated by the opportunistic weedsEpilobium sarmantaceumandRumex crispus. Upstream,Carex gaudichaudianaandBaeckea gunnianadominated. Marked seasonal changes in vigour and biomass were recorded. Plants, but not sediments, downstream of the inflow contained more nutrients than those upstream. Seed bank analysis showed more seeds downstream than upstream. A moist treatment promoted more seed germination than a flooded treatment.E. sarmantaceumand to a lesser extentR. crispusdominated in each treatment. In an effort to promote greater water retention and replacement of the weed species, channels in the wetland have been blocked with gravel and planted withPhragmites australisandSchoenoplectus vali