We have measured the temperature and magnetic field dependence of the resistance for bulk samples of single-walled nanotubes treated in different ways. Hydrostatic pressure-treated samples and as-grown samples show a two-dimensional variable-range hopping behaviour while samples treated in non-hydrostatic pressure or in acid for a long time show a three-dimensional transport behaviour. We suggest that the observed two-dimensional behaviour arises from a network structure with a fractal dimension lower than three.