Slips are likely to wreak havoc across New Zealand’s road network unless the response to disasters changes, a travel network expert claims.

Motorists were forced to sleep in their cars after trees blocked State Highway 1, while Kāwhia was isolated because of multiple slips as a result of wet weather.

Dr Douglas Wilson, director of the Transportation Research Centre at the University of Auckland, said the age and design of New Zealand’s roads can make them susceptible to slips.

“The road networks in New Zealand obviously have largely been built sort of 50 to 100 years ago and a lot of them were not actually designed to current standards or even to standards at that point because they followed the secondary road network,” Wilson said.

He explained that the geological materials the roads were founded on were more moisture sensitive because they hadn’t gone through “lithification”, the landscape is steep and extreme weather is happening more often, combining to make the drainage systems work over capacity regularly.

“All of these things combined together to make it much more susceptible in a lot of our road networks around the country creating significant problems during these events.”

Slips and flooding have blocked state highways across the North Island after extreme wet weather events. Photo / NZTA

Wilson said it wasn’t possible to completely rethink the country’s road network, largely because of cost, but a targeted approach could help improve resilience.

“I’m a strong believer in a strategy designed approach, which is trying to get in front of the wave rather than being reactive.”

Wilson said by targeting and highlighting certain regions that are more at risk, the road network won’t have to be under regular repair and would be effective without a huge increase in spending.

“We have to do a nuanced prioritised targeted approach to improvements over time because we can’t afford to do it overnight and we are getting hit on all sorts of fronts.”

A NZ Transport Agency Waka Kotahi spokesperson said they understand recent weather and its affects have generated significant interest in the transport network and its resilience.

They said significant infrastructure investment has already happened, with about $1 billion spent on responding to weather in 2023 and repairs and improvements from these have performed well in bad weather this year.

The spokesperson also said proactive maintenance is done to improve the state highway network reliability.

They said NZTA uses a range of tools to identify natural hazards on the state highway network and assess their risk and prioritise investment.

A 30-year plan is also being used to strategise planning, development and investment in the land transport system.