The modified permeability of an idealized unidirectional reinforcement consisting of regularly ordered, parallel fibres in the resin transfer moulding process is derived for a power-law fluid flow perpendicular to the fibres. An approximate expression based on the lubrication approximation is obtained starting from first principles. In addition, an expression for the permeability, when the resin exhibits a Newtonian behaviour, is derived in the same way as for the Gebart model. The modified permeability is expressed in terms of Newtonian permeability and porosity in order to avoid explicit reference to the fibre radius in the final expression. Thereby, the fibre radius need not be determined in order to calculate the modified permeability. Finally, the validity range of the analytical derivations in terms of the power-law index, n, and the fibre volume fraction of the two models are considered by CFD-simulations. With help from numerical results a correction of the approximated solution is also made in order to get agreement to a greater extent for more extreme values of n.