Study region: The Northern Murray-Darling Basin, Australia. Study focus: We estimated the impacts of meteorological trends on streamfiow over a 40-year period for seven catchments in the Northern Murray-Darling Basin (NMDB), Australia. New hydrological insights for the region: We found that meteorological trends over the 1981-2020 period explain all the streamfiow decline in catchments with little or no irrigation withdrawals, whereas in catchments with substantial irrigation water withdrawals meteorological trends explained only about half the observed decline in streamfiow. If the increase in water withdrawals for irrigation over the 2006-2020 period relative to 1981-2000 had, instead, been reallocated to mitigate declines in streamfiow over the 1981-2000 period, the average annual gross value of irrigated agriculture (GVIA) in the NMDB would have been reduced by 9-14 %. If a water reallocation over the 2006-2020 period had been undertaken to maintain the same mean ratio of irrigation water withdrawals to streamfiow over the 1980-2000 period, GVIA would have declined by 19-29 %. Our results highlight the importance of quantifying and partitioning the effects of long-term meteorological trends on streamfiow in semi-arid and arid environments to improve water planning and allocation.