In this paper, modelled hydrological data are used to quantify the effects of regulation on the flow regime of the lower Murrumbidgee River in the period 1970-1998. Although other studies report historical changes in flood frequency and duration, this study uses modelled natural daily flow data rather than pre-regulation records or aggregated modelled monthly data. The comparison of modelled natural and regulated daily flows shows the magnitude of changes to mean and seasonal flows, flood peaks and flow duration. At gauges upstream of major irrigation off-takes, mean flows have been increased by approximately 10 per cent, flood peaks have been reduced by 21-46 per cent, and there has been a seasonal redistribution such that flows in summer and autumn have been increased at the expense of those in winter and spring. At gauges downstream of the major irrigation off-takes, mean flows have been reduced by 8-46 per cent, flood peaks have been reduced by 16-61 per cent, and flows have been decreased in all seasons.