It could be months before Wellingtonians are given the all-clear for swimming at southern coastal areas - where sewage is leaking

Wastewater's backed up a pipe at the Moa Point treatment plant - flooding its lower floors.

Wellington Water chief executive Pat Dougherty says diverting the spillage will slightly improve water quality - but people shouldn't swim or fish until the plant's operating as usual.

He says we won't know how long that will take until the wastewater's cleared and we know the scale of damage to equipment.

Dougherty says some will have to be imported, so it could take months.