A system comprising: a sensing device comprising a pressure sensor configured to measure pressure of water in a water system of a structure during a testing interval when a water system shutoff valve of the water system is closed, wherein the sensing device is configured to generate pressure measurement data representing the pressure of the water as measured by the pressure sensor; and one or more processing units (310, 404, 1310) comprising one or more processors and one or more non-transitory storage media storing machine-executable instructions, the system characterised by the one or more non-transitory storage media storing machine-executable instructions being configured when run on the one or more processors to perform: estimating an estimated orifice diameter of a leak in the water system based on the pressure measurement data by averaging diameters calculated based on repeated pressure sampling throughout the testing interval, wherein: the diameters calculated are based on: d = ‖ P t − 1 − P t × A ρ × g × Δt × k × 2 × P t − 1 ρ ‖, where d is a calculated diameter of the diameters calculated assuming (i) the water system is adiabatic, (ii) constant flow rate from the leak, (iii) the leak is approximated as a circular area, and (iv) no consideration of friction losses, P t- 1 - P t are two pressure samples of the pressure measurement data at two successive times within the testing interval, A is a cross-sectional area of piping in the water system, ρ is a density of the water, g is a gravitational acceleration constant, Δ t is a time differences between the two successive times, and k is a dimensionless flow coefficient set to 1; and the testing interval is approximately 10 minutes to approximately 20 minutes. a sensing device comprising a pressure sensor configured to measure pressure of water in a water system of a structure during a testing interval when a water system shutoff valve of the water system is closed, wherein the sensing device is configured to generate pressure measurement data representing the pressure of the water as measured by the pressure sensor; and one or more processing units (310, 404, 1310) comprising one or more processors and one or more non-transitory storage media storing machine-executable instructions, the system characterised by the one or more non-transitory storage media storing machine-executable instructions being configured when run on the one or more processors to perform: estimating an estimated orifice diameter of a leak in the water system based on the pressure measurement data by averaging diameters calculated based on repeated pressure sampling throughout the testing interval, wherein: the diameters calculated are based on: d = ‖ P t − 1 − P t × A ρ × g × Δt × k × 2 × P t − 1 ρ ‖, where d is a calculated diameter of the diameters calculated assuming (i) the water system is adiabatic, (ii) constant flow rate from the leak, (iii) the leak is approximated as a circular area, and (iv) no consideration of friction losses, P t- 1 - P t are two pressure samples of the pressure measurement data at two successive times within the testing interval, A is a cross-sectional area of piping in the water system, ρ is a density of the water, g is a gravitational acceleration constant, Δ t is a time differences between the two successive times, and k is a dimensionless flow coefficient set to 1; and the testing interval is approximately 10 minutes to approximately 20 minutes.