Why Wellington Water didn’t spot big leak

The leaking pipe that ate a hole under a Wellington street and saw the city come close to running out of water was not picked up because it was not big enough to trigger an alarm.

When discovered, a bit less than 15 litres per second was being lost, but in the months leading up to it being noticed it was likely to be significantly less, Wellington Water manager of network and customer operations Gary O’Meara said.

Although there was ‘‘a lot of water,’’ being lost it ‘‘was insignificant in comparison to the total water being supplied to Wellington and being consumed’’, O’Meara said.

The city’s main supply pipe was turned off from Friday night to Saturday morning so temporary repairs could be carried out on a leaking valve under Featherston St.

The incident was a reminder of the vulnerability of the city’s pipe systems in an earthquake, O’Meara said.

The company had a number of measures in place to monitor water levels, pressure and flow in

The leak "was insignificant in comparison to the total water being supplied to Wellington and being consumed". Gary O'Meara, Wellington Water

the city’s reservoirs and pipes.

Monitoring included a 24-hour real time data acquisition system that recorded reservoir levels, flows in and out of reservoirs and pressure in pumping stations and pipes.

An alarm was attached to the system and would send out an alert if parameters were breached, but that was not likely unless there was a major burst, O’Meara said.

Pipes in Wellington’s CBD were also checked physically by people twice a year, using listening devices designed to pick up any sign of a leak.

The alarms did not go off for the breach under Featherston St because the amount of water spilling out of the leak ‘‘was insignificant’’.

O’Meara could not say the exact amount of water lost from the pipe but said ‘‘over a period of time there were thousands of litres coming out’’ of it.

O’Meara could not say when the pipe in question was last checked, but said checks on buried pipes were carried out based on age, type of pipe, installation and maintenance history.

‘‘It is not as if that pipeline was due for renewal.

‘‘It has plenty of years left,’’ O’Meara said.

‘‘An external force has caused damage rather than the failure of the pipe.’’