Excess nutrient loads conveyed in stormwater runoff are largely responsible for eutrophication of receiving waters. Bioretention systems are a stormwater treatment device designed to reduce nutrient loads. Whilst bioretention systems are very effective in filtering particulate nutrients in runoff, they are less effective for long term retention of dissolved nutrients. Since dissolved phosphorus (P) and nitrogen (N) often comprise a substantial fraction of the runoff nutrient loads, this results in lower overall nutrient retention. This research investigated the effectiveness of N and P removal from recycled effluent and synthetic stormwater using different filter media and flow control outlets. Methods Two series of mesocosm experiments were conducted (Experiments 1 and 2). The bioretention mesocosms were 240L containers (0.25 m2 area, 1.0 m deep), incorporating different media planted with 5 plants. Three different media (Expt. 1) and 7 different media (Expt. 2) were investigated for their P retention properties. To simulate exposure to long-term nutrient loads, loading regimes applied recycled tertiary treated effluent weekly. Every six months, a dosing regimen comprising a series of synthetic stormwater runs was conducted. Outlets were installed to control retention time in the different media treatments in all treatments but one. Two treatments were unvegetated to examine the effect of plants.