The invention relates to a multi-stage hydraulic fracturing tool and system for controllably exposing selected locations along a wellbore to a pressurized fluid. The system comprises an elongated casing (for disposal within the wellbore) defining an internal borehole extending longitudinally, and having one or more ports; an actuation member configured for travelling down the borehole and includes a wedged portion and a groove having a first length in the longitudinal direction, formed at least partially circumferentially around an outer surface of the actuation member, a sliding sleeve member having an aperture for receiving the actuation member, and one or more inward-facing protrusions having a length less than or equal to the first length, connected to the sliding sleeve member and at least initially protruding radially into the aperture.