The present invention relates to a tap assembly for dispensing fluids from a flexible film bag such as a “bag-in-box”. Known taps associated with such flexible bags do not efficiently dispense fluid from a bag which is partially collapsed and/or can damage the opposite wall of the partially collapsed bag. The tap assembly includes a body with a bore, an actuation member to cover at least a portion of the bore and a piercer which is configured to pivot within the bore and pierce and cut a sterility membrane positioned over an open end of the bore upon movement of the actuation member by a user. The piercer comprises a curved surface which functions to position the piercer away from the plane of the adjacent bag wall after initial piercing.