Heavy rain to test Wellington's infrastructure

Wellington City Council expected heavy rain to hit Wellington and Porirua until as late as Thursday afternoon. Photo / iStock

Sunnier skies are expected for many next week as a big new weather system moseys across the country, but one city has been warned of more heavy rain in the next day.

Heavy rain overnight would be another test for Wellington's infrastructure, which was no match for a downpour yesterday.

A big high pressure system was heading for New Zealand.

The high would take about a week to cross the country, bringing calmer, cooler, drier and sunnier weather to many places, WeatherWatch said.

But WeatherWatch forecasters said a few showers were expected before the high arrived.

The forecasters said no "aggressive lows", thunderstorms or hail were expected anywhere next week.

That prediction should perk up Wellingtonians who felt the impact of this week's flooding.

But first, a rainy night was expected.

MetService said showers in Wellington would turn to rain this evening. Strong northerly gusts of up to 100km/h were forecast.

Wellington City Council expected heavy rain to hit Wellington and Porirua until as late as tomorrow afternoon.

The situation prompted the council to implore residents to keep the city's drains clear.

A combination of fallen autumn leaves and detritus from yesterday's dumping meant council contractors could not be guaranteed to keep all drains clear, the council revealed in a statement this evening.

"It'd be great if residents in flood-prone areas can check if drains on or outside their properties are clear of autumn leaves or other debris," a council spokesman wrote.

"If residents can run a broom over any blocked sump then we'd appreciate your help."

The country's capital yesterday warned its infrastructure was too old and flimsy to cope with heavy rain.

The council said two major downpours in as many weeks were beyond the capacity of the city's drainage network. Much of that network was more than a century old.

Environment committee chair Iona Pannett said the council faced flooding "hot spots" especially around low-lying areas.

These danger zones included the Basin Reserve area, Kilbirnie, lower Aro Valley and parts of the CBD.

"We have been spending millions of ratepayers' dollars in the past couple of decades to increase stormwater capacity in areas like Island Bay, Karori, Tawa and the Golden Mile," Ms Pannett said.

"Now it's clear we have more problem areas to deal with..."

The councillor said drainage network capacity, not blocked drains and sumps, was the main cause of the flooding.

She hoped to tackle some of these problems when the council started tailoring its 10-Year Plan.

Climate change and projected sea-level rises were also playing a part in the planning.

Ms Pannett said drainage network capacity, not blocked drains, was largely to blame for Tuesday's flooding.

Although some inexpensive short-term flood prevention steps could be taken, she said new pump stations or larger stormwater mains may have to be considered for a long-term fix.

Wellington Mayor Celia Wade-Brown said older drains were a concern.

This was despite the city having budgeted more than $150m in operational spending and at least $56m in capital spending over the next decade for stormwater assets alone.

"The combination of some older drains, more impermeable surfaces and increasing rainfall severity means that system modelling is more and more important," the mayor said.

"For the longer term we must design our streets and sections to be more permeable and add more planting."