The deleterious effect of water is a major factor that causes early degradation in hot mix asphalt (HMA) materials. The coefficient of permeability (k) is a parameter used to characterise HMA hydraulic transport properties. This study numerically evaluated the influence of the spatial distribution of k on the flow pattern within an asphalt surface layer with different thicknesses. Initially samples with different heights were prepared in the lab and cut in three parts and the air voids (AV) content and k values in each part were determined. From the experimental relationship between AV and k numerical simulations were conducted to characterise the transient flow inside an asphalt surface layer divided into sublayers in order to represent typical AV vertical distributions observed in the field. The results show that the heterogeneous distribution of k within the HMA layer could produce regions that are more susceptible to moisture damage due to the accumulation of water.