Many urban river systems are impacted by instream barriers which present significant obstacles for diadromous fish migration. Australian bass, Macquaria novemaculeata, typically occupy freshwater environments but make annual migrations to estuaries to spawn. In the Lane Cove River, Sydney, a weir demarcates the upper tidal limit and bass must overcome it to make their breeding migration. Here we examined the movement of bass in the river using acoustic technology. We show that half the tagged fish successfully migrated both downstream and returned upstream multiple times during the tagging period. Some migrated as far as the Sydney Harbour Bridge, which is very near pure sea water. Downstream migration was linked with heavy rainfall, whereas the return migration was associated with high tides. We suspect the fish return to the freshwater reaches by using a newly built fish ladder that is only accessible at high tide. Our data show that high water flow is a key environmental driver of spawning behaviour in this species and that most of the spawning behaviour occurs in June within the estuary near the mouth of the river.