Top quark production in the process e + e - → t t ¯ at a future linear electron positron collider with polarised beams is a powerful tool to determine indirectly the scale of new physics. The presented study, based on a detailed simulation of the ILD detector concept, assumes a centre-of-mass energy of s = 500  GeV and a luminosity of L = 500 fb - 1 equally shared between the incoming beam polarisations of P e - , P e + = ± 0.8 , ∓ 0.3 . Events are selected in which the top pair decays semi-leptonically and the cross sections and the forward–backward asymmetries are determined. Based on these results, the vector, axial vector and tensorial CP conserving couplings are extracted separately for the photon and the Z 0 component. With the expected precision, a large number of models in which the top quark acts as a messenger to new physics can be distinguished with many standard deviations. This will dramatically improve expectations from e.g. the LHC for electro-weak couplings of the top quark.