A portable water tank assembly for a dust emitting construction machine, such as a machine for cutting concrete, stone, ceramic materials and the like, said assembly comprising a water container (1) and an electric pump (2) powered by a battery for pumping water from the container (1) through an outlet to the construction machine. To provide a portable water tank assembly which provides a long service life without changing battery, is easy to handle and carry and that is robust and reliable, the pump is a low pressure pump (2), of 0.1-1.5 bar, preferably of 0.3-1.2 bar, that is preferably submersible and placed inside the water container (1); and said assembly has at least 5 liter capacity. Preferably the hood member (3) has a first compartment (5) for a battery for delivering power to the pump (2) and a battery (6) located in the compartment (5), and it may have a second compartment (11) for a battery charger (12) and a third compartment (13) for a spare battery.