Time-domain modelling of wave-energy devices is important. This is due to the need for information on the device’s transient response characteristics; even when linear potential theory is assumed when modelling hydrodynamic loads, significant non-linearities may be present in the system due to the power-take off (PTO), mooring, and control subsystems. In this paper, an approach for modelling multibody marine systems is presented. The Newton-Euler equations with eliminated constraints (NE-EC) are utilised to capture the rigid body dynamics of the constrained multibody system. This results in the convenient integration of active loads (as opposed to interbody constraint forces) acting on the multibody system. In this paper, the active loads considered are: hydrodynamic, PTO, and mooring loads.