A portable water tank assembly for a dust emitting construction machine, such as a machine for cutting concrete, stone, ceramic materials and the like may include 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. The assembly has at least 5 liter capacity.