Residents in a wide area of central-west Auckland - including Westmere, Grey Lynn, Cox's Bay, Western Springs, Great North Rd, Ponsonby, Newton and Arch Hill - may notice low water pressure and some discolouration.
However, the water was safe to use and drink, Watercare said.
No water outages were expected but residents are asked to reduce water consumption.
"Our contractors arrived on site within an hour of being notified at 6.45am," the council-controlled organisation said.
"They will begin repairs once the broken section of pipe has been isolated. Several water valves are located along Tay St, Great North Rd and Ivanhoe Rd and traffic management is in place."
Engineers were currently on site and working to repair the fault, the Watercare website said.
The current restoration is expected to be completed by midday on Tuesday.
Updates will be available on the Watercare website.