Packer test data and tunnel inflow observations are used to show that the distribution of equivalent hydraulic conductivity obtained for a rock mass from borehole packer testing will be biased if the orientations of the planned tunnel and testing borehole differ. Furthermore, test results show that the distribution of equivalent hydraulic conductivity is different in various geologic sub-units at the same site. The impact on inflow of the major uncertainties, including conductivity, head and specific yield, is illustrated to be amenable to a combined analysis using Monte Carlo simulation.