To restore streams degraded by urbanisation, bioretention basins ('raingardens') are implemented worldwide to reduce stormwater volumes, peaks flows and pollutant loads entering streams. While they are effective at this, their ability to mimic natural flow regimes has received limited field investigation. We monitored the hydrology and water quality of a bioretention basin during a three-year period. We found that the basin reduced the delivery of polluted water to the stream (55-65% of catchment runoff released as treated outflow), with peak flows and event concentrations of TSS, TP and TN substantially lowered too. A smaller proportion of catchment runoff infiltrated into the surrounding soil, but the flow regime delivered by the basin to the local stream was not similar to nearby forested streams. Such systems can mitigate the impacts of urban runoff on streams and shift the urban water balance towards natural conditions, but complete flow regime restoration requires catchment-scale action.