The redshifted 21 cm line signal is a unique probe for the Epoch of Reionisation (EoR)—it enables tomographic studies, which track the evolution of the averaged intergalactic medium (IGM) properties and their fluctuations. Accurate modelling of the 21 cm global and power spectrum signals is crucial for interpreting measurements from current and forthcoming 21 cm experiments. Theoretical predictions usually post-process reionisation simulations with optical depth approximation, which treats local line broadening and peculiar velocities approximately and often leads to divergences due to its velocity gradient term. I will present our cosmological 21 cm line radiative transfer (C21LRT) formulation, which explicitly accounts for local 21 cm line emission and absorption, Doppler shifts by peculiar velocity, broadening of the line, and the radiation transfer effects. We will adopt the IGM properties from reionisation simulations as inputs and assess the accuracy of the optical depth approximation for predicting 21 cm global signals and power spectra. I will demonstrate how our C21LRT delivers robust results for the redshift-space distortion (RSD) effects in the 21 cm power spectrum. I will show where the optical depth approach remains valid (for length scales of 1 -10 cMpc at mid-reionisation) and highlight the need to fully quantify the uncertainties with further exploration across a wider EoR parameter space using our C21LRT code.