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Lake Parramatta was a disused water storage located at the bottom of an 8 km2 urbanised watershed in Sydney, Australia in which primary contact recreation had been actively discouraged, because of the perceived risk of infection from waterborne pathogens. However, a recent survey of the Lake showed that safe swimming might be possible during summer dry weather periods, providing high contamination periods (during rain events) could be identified and avoided. To this end we characterized inflow/lake water quality (bacterial indicators, fecal sterols, Cryptosporidium, viruses & turbidity) and hydrology (rainfall, streamflow & Lake depth measurements) to quantify the cycle of lake contamination and water quality recovery. Fecal contamination became noticeable for rainfall inputs > 10 mm in < 24 h, and this threshold was adopted as a provisional trigger for the management of high risk periods. Lake recovery to background indicator concentrations (<10 enterococci.100mL-1) occurred within 3 d after a 24 h rainfall of 70 mm. Solar irradiance was identified a likely key inactivation factor (typically > 25 MJ.m-2.d-1, with 10% visible radiation reaching 1 m). None of a range of enteric viruses was detected in the Lake immediately after high stormwater inputs. Fecal sterol analysis indicated that sewage only represented some 0.1% of run-off into the Lake. This data was used to develop a water safety management plan for efficient and timely recreator protection from pathogen-related events, and the Lake was reopened in December, 2006.