The performance of air-cooled geothermal power plants is susceptible to the changes in ambient temperature especially in New Zealand due to the large difference in the atmospheric temperature for summer and winter seasons. The turbine is typically designed for a specific set of operating conditions, but the operating conditions also change with the resource enthalpy and the heat sink temperature. The effect of the fluctuation in the condenser temperature due to the changes in ambient temperature affects the pressure ratio across the turbine, thus reducing the efficiency of the turbine. This paper investigates the potential for adapting a 100 kW gas turbine with fixed nozzle vanes into an ORC turbine. An air-cooled ORC system was designed using the geothermal source conditions and the mean daily temperature in Rotokawa, New Zealand. The effect of the air temperature on the condenser temperature, and thus on the operation of the turbine was studied. The performance of the turbine was studied via computational fluid dynamic (CFD) tools with different pressure ratio to take into account of subsonic expansion and supersonic expansion across the selected turbine in a single stage configuration. The turbine performance curve was then incorporated into the ORC system and the efficiency map of the ORC system using the common fluids, namely isobutane, R245fa and R134a, was generated. The model results were used to develop a correlation between the ambient temperature and the performance of the ORC system.