A turbine assembly for a turbocharger, comprising: a turbine housing defining a bore extending along a downstream axial direction and defining a chamber surrounding the bore for receiving exhaust gas, and a nozzle (65, 70) leading from the chamber generally radially inwardly into the bore; a turbine wheel rotatably mounted in the bore and comprising a hub and a plurality of turbine blades extending generally radially outwardly from the hub and terminating in blade tips, the hub of the turbine wheel and the bore of the turbine housing being configured such that exhaust gas flowing from the nozzle to and through the turbine wheel undergoes a change in direction from a generally radially inward direction in the nozzle to the downstream axial direction downstream of the turbine wheel; a generally ring-shaped divider in the nozzle dividing the nozzle into a first-stage nozzle and a second-stage nozzle, the first-stage nozzle being delimited between a fixed member of the turbine assembly and the divider, the divider having an upstream surface guiding exhaust gas flowing through the first-stage nozzle and an opposite downstream surface guiding exhaust gas flowing through the second-stage nozzle, the divider having a radially innermost edge; and a tubular piston disposed in the bore of the turbine housing and being axially slidable therein, the piston having a radially inner surface extending along the axial direction and having a non-flat upstream end surface, with a first radius, that includes a contact region for contacting a portion of the divider with the piston in a closed position so as to substantially close the second-stage nozzle, and movement of the piston in the downstream axial direction opening the second stage nozzle, characterized in that the piston includes a curved flow guiding surface with a second radius proceeding from the contact region to the inner surface, the flow guiding surface turning from a generally radially inward direction to the downstream axial direction. a turbine housing defining a bore extending along a downstream axial direction and defining a chamber surrounding the bore for receiving exhaust gas, and a nozzle (65, 70) leading from the chamber generally radially inwardly into the bore; a turbine wheel rotatably mounted in the bore and comprising a hub and a plurality of turbine blades extending generally radially outwardly from the hub and terminating in blade tips, the hub of the turbine wheel and the bore of the turbine housing being configured such that exhaust gas flowing from the nozzle to and through the turbine wheel undergoes a change in direction from a generally radially inward direction in the nozzle to the downstream axial direction downstream of the turbine wheel; a generally ring-shaped divider in the nozzle dividing the nozzle into a first-stage nozzle and a second-stage nozzle, the first-stage nozzle being delimited between a fixed member of the turbine assembly and the divider, the divider having an upstream surface guiding exhaust gas flowing through the first-stage nozzle and an opposite downstream surface guiding exhaust gas flowing through the second-stage nozzle, the divider having a radially innermost edge; and a tubular piston disposed in the bore of the turbine housing and being axially slidable therein, the piston having a radially inner surface extending along the axial direction and having a non-flat upstream end surface, with a first radius, that includes a contact region for contacting a portion of the divider with the piston in a closed position so as to substantially close the second-stage nozzle, and movement of the piston in the downstream axial direction opening the second stage nozzle, characterized in that the piston includes a curved flow guiding surface with a second radius proceeding from the contact region to the inner surface, the flow guiding surface turning from a generally radially inward direction to the downstream axial direction.