Whangārei Mayor Vince Cocurullo says the city would cope reasonably well if an Auckland-style rain deluge hit.

His comments come as Tropical Cyclone Gabrielle, labelled by one New Zealand weather forecaster as one of the “most serious storms of the century”, potentially bears down on the top of the North Island.

Auckland Anniversary weekend’s one-in-200-year rainfall event saw four people die, nearly 2000 houses damaged badly enough to have access officially restricted and more than 1500 cars inundated with floodwaters.

So far, there have been more than 270 Auckland houses red-stickered and more than 1600 houses yellow-stickered. Auckland Airport recorded 245 millimetres of rain on Friday, January 27, its wettest day – part of Auckland’s wettest January since records began in 1853. Auckland’s Māngere weather station received 859 per cent more than its normal rainfall for the month.

Cocurullo, who is also the Whangārei District Council (WDC) representative on the Northland Civil Defence Emergency Management Group, said the district would cope reasonably well if such a deluge happened here. There were, however, many variables that would influence exactly how well it coped.

These included the rainfall’s geographic coverage, how localised it was, the ground’s degree of soil saturation, how much rain had fallen previously and the timing of rainfall in relation to high tides.

He said Whangārei was not Auckland and the city had a lower population density than Auckland. Whangārei was less densely urbanised, with more parks per head of population. More green spaces meant the land was more permeable, making it better able to absorb falling rain, rather than the rain simply running off over concrete and buildings.

Cocurullo said there had been a lot of infrastructure work done towards building Whangarei’s resilience, but that there was naturally “always room for improvement”.

“There are a lot of things we’re still working on,” Cocurullo said.

When asked how well Whangārei’s stormwater infrastructure was set up to cope with the demands of a changing climate’s rainfall and more intense bursts of heavy rain, Cocurullo said the network was up to scratch. It almost fully met existing required national standards, save for a couple of older areas.

The council had recently updated its requirements around stormwater provision through new environmental engineering standards. He said the time might be coming for New Zealand’s stormwater drainage requirements to be reviewed in the face of changing weather. Investigation into tropical Queensland’s stormwater infrastructure provision was an option for informing future options for New Zealand regarding this Three Waters aspect.

He cited working through different locations, on a catchment-by-catchment basis, checking the pipes in each and whether they were up to selected specific rainfall intensities as an option. That meant asking whether a specific catchment’s stormwater infrastructure could handle a particular amount of rain.

Cocurullo said WDC had been working through making sure stormwater and wastewater were not carried together through the same Three Waters infrastructure pipes to reduce the risk of huge amounts of major rainfall from a storm getting into the city’s wastewater treatment system and cut back sewage contamination in any overflowing water.

He said Whangārei’s $11 million Kotuku detention dam Hopua te Nihotetea, in Maunu, had significantly cut Whangarei’s city centre flooding since it opened in 2016.

Resilience work also included working on making sure the district’s rural roads were up to a standard where they weren’t blocked by slips so they could continue to be used in a flooding emergency.

Meanwhile, when asked whether people should be allowed to build in flood and coastal hazard zones, Cocurullo said the council could not stop people building in these locations.

The updated district-wide flood and coastal hazard mapping predicts sometimes dramatically increased areas of flooding into a climate change-challenged future for Whangārei and Northland.

Cocurullo said the council could only encourage people not to build in flood hazard zones. If they chose to do so, it could only stipulate that all standards, including environmental engineering and building standards, were met. Government direction on this would need to be changed through national policy statements which underpinned what the council could do.

Whangārei’s 489mm January rainfall was the district’s highest level of rainfall ever for the month and more than six times its monthly average since records began in 1937.

■ Local Democracy Reporting is Public Interest Journalism funded through NZ On Air