A diffuser for a jet pump of a separator for separating contaminants from a fluid stream which includes entrained contaminants, the diffuser comprising: an inlet portion having an inlet which is adapted to receive a fluid stream including residual entrained contaminants, the inlet defining a first flow area; an outlet portion in fluid communication with the inlet portion and having an outlet through which fluid exits the diffuser, in which a flow path for the fluid stream is defined which extends from the inlet to the outlet, and in which the outlet defines a second flow area which is greater than the first flow area so that a velocity of fluid flowing through the inlet is greater than a velocity of fluid flowing through the outlet, the outlet describing a maximum internal dimension; a transition portion disposed between the inlet portion and the outlet portion, the transition portion defining a third flow area which is greater than the first flow area but less than the second flow area, and an opening describing a maximum internal dimension; and a communication port extending through a wall of the diffuser and having an inlet which communicates with an interior of the diffuser and an outlet which communicates with an exterior of the diffuser, in which the inlet of the communication port is located at a position along the flow path which is between the inlet of the diffuser and the outlet of the diffuser, so that contaminants which have become separated from the fluid stream during flow along the flow path can be removed from the diffuser through the communication port; in which the maximum internal dimension described by the opening of the transition portion is less than the corresponding maximum internal dimension described by the outlet of the outlet portion; and characterised in that the transition portion comprises a lip which describes the maximum internal dimension of its opening, the lip being disposed in the flow path, an internal bore of the diffuser stepping up from the maximum internal dimension described by the lip to the maximum internal dimension defined by the outlet of the outlet portion. an inlet portion having an inlet which is adapted to receive a fluid stream including residual entrained contaminants, the inlet defining a first flow area; an outlet portion in fluid communication with the inlet portion and having an outlet through which fluid exits the diffuser, in which a flow path for the fluid stream is defined which extends from the inlet to the outlet, and in which the outlet defines a second flow area which is greater than the first flow area so that a velocity of fluid flowing through the inlet is greater than a velocity of fluid flowing through the outlet, the outlet describing a maximum internal dimension; a transition portion disposed between the inlet portion and the outlet portion, the transition portion defining a third flow area which is greater than the first flow area but less than the second flow area, and an opening describing a maximum internal dimension; and a communication port extending through a wall of the diffuser and having an inlet which communicates with an interior of the diffuser and an outlet which communicates with an exterior of the diffuser, in which the inlet of the communication port is located at a position along the flow path which is between the inlet of the diffuser and the outlet of the diffuser, so that contaminants which have become separated from the fluid stream during flow along the flow path can be removed from the diffuser through the communication port; in which the maximum internal dimension described by the opening of the transition portion is less than the corresponding maximum internal dimension described by the outlet of the outlet portion; and characterised in that the transition portion comprises a lip which describes the maximum internal dimension of its opening, the lip being disposed in the flow path, an internal bore of the diffuser stepping up from the maximum internal dimension described by the lip to the maximum internal dimension defined by the outlet of the outlet portion.