This paper quantifies the impacts of climate variability and plantation expansion on streamflow in Crawford River catchment, Australia using data from 1982 to 2006. The catchment is about 700 km2, with mean annual rainfall of about 750 mm, and Tasmanian blue gum (Eucalypts globulus) plantations were introduced gradually in around 26% of the catchment starting from 1998 until 2005. A revised version of Xinanjiang model which uses measured vegetation time-series data to calculate actual evapotranspiration is used. First, the model is calibrated and validated by using meteorological data and leaf area index (LAI) data from 1982-1997 (pre-treatment period). Then, the model is used to simulate monthly streamflow in a testing period from 2000 to 2006 to estimate the effects of plantation expansion on streamflow (with the LAI value fixed to the same value as in the pre-treatment period of 1982-1997). The difference between measured streamflow in the pre-treatment period and the testing period (36.5mm, -51% of pre-treatment) reflects the combined impact of climate variability and plantation expansion in the catchment. The difference between simulated and measured streamflow in the testing period (18.7mm, -26% of pre-treatment) reflects the impact of plantation expansion in the catchment.