An investigation on the unsteady hydrodynamic behaviour of a new scale model tidal turbine at the University of Auckland has been conducted, using FAST software from National Renewable Energy Laboratory. The turbine blades are designed to achieve the optimal angle of attack based on maximizing lift-to-drag ratio, and then the distribution of chord lengths and pitch angles is determined. In this way, a highly efficient but a lightly loaded tidal turbine model design utilizing the S814 profile, is achieved. Oscillatory motions of flows with single frequency over a range of current numbers and reduced frequencies have been considered. The investigation shows that the unsteadiness of the hydrodynamics on the blades is governed by the dynamic inflow phenomenon. The mean angle of attack along the blade is maintained at or very near the initial optimal value. The effects of the oscillatory frequency and velocity amplitude on the unsteady hydrodynamic loading are explored. A quasi-steady analysis is used to clarify the hydrodynamic response of the model as a lightly loaded rotor. A mathematical model is added to the investigation to analyse the role of the forcing components relative to oscillatory velocity and acceleration on the phase of the unsteady loading.