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.