A counter-rotating hydraulic micro-turbine has been developed to recover the energy lost in release valves of water supply network. This compact two-stage axial turbine could be installed in line with the conduit, reducing the infrastructure investments, which is a crucial point for small power plant. The two stages of the turbine allow to use this axial turbine in case of high head conditions. Steady numerical simulations of the flow in the turbine have been carried out using ANSYS CFX to develop the design of the turbine and assess its performance. The influence of numerical set up such as the mesh size and the type of interface between the stator and the two rotating parts has been studied. The influence of the elbow and the honeycomb on the inflow conditions was also assessed. A best efficiency of 85 % is reached when the two runners counter-rotate at the same nominal speed. When the turbine operates under part and full load conditions, the relative rotational speed between the two runners will allow optimizing the efficiency.