A machine for working a surface with a high-pressure water jet, comprising a mobile chassis (11) with a boom (14) that carries a lance (22) with a nozzle (23) for spraying water, and a drive unit (35) that comprises a motor and control units for the chassis and the boom. The chassis (11) comprises an underframe (33) and a carrier unit (34) shiftably mounted on the underframe, which carrier unit carries the boom (14) and the drive unit (35) so that the machines center of gravity can be shifted by shifting the carrier on the underframe.
