Residents warned to prepare as heavy rain starts to drench lower half of North Island

Heavy rain is starting to fall in the lower half of the North Island as regions brace for a drenching that could spell more trouble for slip-prone areas.

MetService is warning heavy rain is due to set in across the bottom of the island today with mountain snowmelt expected to add to possible flooding woes.

More wet weather is approaching the north of the country and likely to saturate northern and eastern regions for the next two days.

Wellington Civil Defence is warning of widespread flooding today as the sodden ground struggles to cope with the coming deluge.

Forecasters say up to 100mm of rain is expected to fall in 16 hours and some intense downpours could hit the region from mid-morning to mid-afternoon.

The Wellington Region Emergency Management Office is on alert, saying there is potential for surface water in Porirua, Hutt and Kapiti. It could affect travel across the region, the office said.

"As the ground is already sodden from last week's rain, there could be some issues - there is the potential for surface water in Porirua, Hutt and Kapiti. It could be a slow commute, so be prepared," the authority posted on its Facebook page.

A major slip already blocking the Ngaio Gorge is not expected to be cleared for up to a fortnight.

Homes were evacuated last week in downtown Wellington after hillsides started to collapse, leaving two properties in peril.

In the past month, slips have blocked a number of other key routes across the region including the Ngauranga Gorge and the Paekakariki Hill Rd.

MetService says 70 to 100mm of rain is expected to fall in the Tararua Ranges throughout the day.

The wet weather stretches up the western side of the North Island. Kapiti Coast, Horowhenua and southern parts of Manawatu can expect problems from flooding and slips.

Cloud blankets the north. Warmer this morning. Colder down south. Rain Warnings for the lower North Island. https://t.co/qHyE5zhh6X ^JR pic.twitter.com/Junmn5k3IF

— MetService (@MetService) July 31, 2017

MetService has also put out a watch for heavy rain on the upper slopes of Mt Taranaki from midday today.

It warned the deluge was likely to lead to snowmelt causing additional run-off into swollen rivers and streams.

Trampers intending to go into southern North Island mountains are being warned to look out for rapidly rising waterways.