A waterjet assembly for an entrainment waterjet cutting head device comprises a passaged component adapted to be connected to a source of pressurised water and a waterjet nozzle for converting water pressure energy to kinetic energy to form a high velocity waterjet. The waterjet nozzle comprises superhard material and has a contracting bore. The passaged component and the waterjet nozzle are arranged such that pressurised water from the source flows through the passaged component to and through the contracting bore in the waterjet nozzle to generate the high speed waterjet. In use of the waterjet assembly, the outlet face of the passaged component is sealingly forced against the upstream face of the waterjet nozzle.