Emergency repair works will be carried out tonight on a 65-year-old cast-iron water main leaking underneath State Highway 1 in Wellington.
The main is on Mana Esplanade at the Acheron Rd intersection.
A metre raised the alarm early this morning that water was leaking through the bulk main.
Wellington Water service delivery manager Vic Maggs said the cause of the failure would become clear during repair work, which is scheduled from 7pm tonight through to 7am tomorrow.
There's currently no risk of the leak getting worse, so repair work has been pushed back until after peak-hour traffic, he said.
"I can't tell you the volume of water that we're losing, but it's not a lot at present. It's remaining underground at the moment so it's not affecting the traffic at all," Maggs said.
The 200ml cast-iron main was installed in 1955.
One northbound lane was closed earlier today to allow crews to assess the leak and determine options for repair.
NZTA has reported there will be an additional brief lane closure before 2pm today for preparatory works.
But all lanes will be open during evening peak-traffic hours, which will be monitored so that work can begin as soon as traffic volumes have sufficiently reduced.
"While the work is underway overnight, the right northbound lane and the right turning bay into Acheron Rd will be closed. Motorists are advised to take extra care when driving through the work site," NZTA said in a statement.
Maggs said there was plenty of water stored in the Plimmerton Reservoir for the next day to supply surrounding businesses and residents with.
A crew of about 15 people will carry out the repair and associated traffic operation tonight, he said.
"We will turn the water off on the main, we will excavate and remove the valve, and put a new valve and connections back in place.
"There are a lot of services under the state highways and local roads so these things do occur from time to time, it's not uncommon. In fact, we have carried out a repair on this asset in the last 10 or so years."