This paper details the experimental results for the influence of transport currents on the ac magnetisation of high-Tc superconductors. It has been found that the variation of the third harmonic amplitude V3 with an ac field can still be described by square and cubic laws for low and high ac fields, respectively, the same relationships as observed in the absence of transport currents. The only significant difference is that the magnitude of V3 in the low field region increases linearly with transport current. Consequently, the crossover point (from square law to cubic law) moves to higher ac fields and the variation of V3 with transport current is less pronounced in the high field region. The results are explained in terms of an effective increase in Hg, which is a parameter in Kim critical state model, with increase of transport current.