New or repaired mains and building water systems can become contaminated during construction, repair or a prolonged period out of use. Effective disinfection begins with hygienic working, identifying the ownership and boundaries of the system, physical cleaning and flushing. A competent team then applies a system-specific disinfection method under the water undertaker's procedures or the building's approved commissioning plan, checks that treatment reaches representative locations, flushes or neutralises the water and verifies results before handover. Public distribution mains and internal building plumbing have different responsibilities, users and approval requirements. Water UK's Principles of Water Supply Hygiene provides authoritative guidance for water undertakers, while HSE HSG274 Part 2 describes building-water commissioning and cleaning. A certificate should identify the asset, method, treatment conditions, test results, limitations and sign-off, but it is not a permanent guarantee: the system must continue to be operated, monitored and maintained. The full article explains how to avoid common failures such as untreated branches, unplanned discharge and stagnation between disinfection and occupation.