This invention relates to an improved vertical axis water turbine assembly (2) for generating rotary power from fluid flow, the water turbine assembly (2) having active blade pitch control. The water turbine assembly (2) comprises a plurality of vertical blades (20) disposed about a vertical assembly axis, each vertical blade (20) having a vertical blade axis and being adapted for movement about said vertical blade axis. The water turbine assembly (2) further includes guide means to control the movement of each of the plurality of vertical blades (20) as the water turbine assembly (2) rotates.