Tension infiltrometers allow water to infiltrate into the soils at various specified pressure heads. The resulting infiltration rates can then be analysed for soil hydraulic properties by either analytical or inverse numerical methods. Tension infiltrometers however are primarily designed to be deployed on horizontal land surfaces, and their applications have been studied widely using an inverse numerical tool HYDRUS-2D. However, natural landscapes are often nonhorizontal, and infiltration through tension infiltrometers on sloped surfaces is no longer an axisymmetrical two-dimensional (2D) process but a fully three-dimensional (3D) one. In addition, minimal research has examined the effect of simplifying the 3D problem to a 2D one on the hydraulic conductivity estimated using tension infiltrometer data from different land slopes of various soil types. Therefore, in this study, tension infiltrometer data on different slopes have been obtained from a catchment located at National University of Singapore. In addition, tension infiltrometer data of six soil types on different slopes and with different initial water content were simulated using HYDRUS-3D. Combining field measurements, forward and inverse modelling, the influence of applying a 2D approximation on hydraulic property estimations using tension infiltrometer data was examined. The results show that the estimations for soils with high infiltration rates are more sensitive to application of the 2D approximation. The maximum allowable slopes for employing 2D approximation on clayey and sandy soils are 25° and less than 3°, respectively. Furthermore, the maximum allowable slope decreases with decreasing initial water content. Copyright © 2012 John Wiley & Sons, Ltd.