The parameterisation of accurate and transferable repulsive potentials is a key ingredient for the self-consistent-charge density functional tight-binding method (SCC-DFTB). In the conventional parameterisation scheme the balanced description of different chemical environments involves significant human effort and chemical intuition. In this work, we propose an in situ parameterisation method with reduced transferability but maximal accuracy for the chemical and physical environment under investigation. Starting from an initial guess, we used iterative Boltzmann inversion to successively improve the repulsive potentials. The corrections were extracted iteratively from the differences in the radial distribution functions with respect to a density functional reference calculation. With this protocol convergence was reached within a few iterations involving only minimal human input. We applied this new scheme to liquid water at ambient conditions, a particularly challenging case for conventional SCC-DFTB. The newly determined parameters lead to a clear improvement of both the structural and dynamical properties of liquid water.