Why State Highway 3 keeps closing in the Awakino Gorge

State Highway 3 through the Awakino Gorge will shut completely for four days from Saturday 22 August so crews can dig the road open and lay three culverts beneath it. It will be the fourth time since the April storm that the road has been closed outright, and by RNZ’s count the eighth closure of the route in recent times.

For the towns along the road, the freight companies that rely on it, and anyone who drives between Taranaki and the Waikato, the closures have stopped feeling like isolated emergencies. They have become the defining fact of the road.

This explainer sets out what is actually failing in the gorge, why engineers cannot simply patch it once and move on, and what the Government’s $50 million resilience commitment is meant to change. Newswire has also reported on what the closure means for Mōkau businesses.

What is closing on SH3 and when

The closure runs from 6am on Saturday 22 August to 6pm on Tuesday 25 August, and it applies day and night for the full four days. NZTA says the road between the rest area near the historic Awakino Tunnel and the intersection with Manganui Road will be an open excavation site, and no vehicles at all, including emergency services, will get through.

The job is the installation of three culverts, the concrete pipes that carry water underneath a road. Since April, water coming off the big slip above the highway has been running across and under the carriageway. The plan is to collect that water at the roadside and carry it under the seal into the Awakino River, rather than letting it soak into the ground the road, and the slip, are sitting on.

Putting culverts in means digging trenches right across the road, and in announcing the closure NZTA incident controller Kendra Ludeke said other options were considered, including managed access, stop and go, and scheduled openings during the work. “This work can only be completed safely under a full planned closure because the culvert installation requires excavation across the road, space for road workers, machinery and materials, and a safe working area within a narrow gorge environment,” she said.

While the gorge is shut, the official alternative between Taranaki and the Waikato is the inland run on SH4 and SH1, which adds hours of driving for a New Plymouth to Hamilton trip. If the weather turns bad enough to stop the work, the whole closure moves back a week, to Saturday 29 August until Tuesday 1 September.

The April storm that broke the road

Everything happening in the gorge this year traces back to a single stretch of heavy rain in mid April. The storm brought down slips all along the route, later counted at about 39 slips and six underslips, after initial reports of more than 30. The largest was a roughly 16,000 cubic metre mass of mud and rock that buried around a hundred metres of highway midway through the gorge.

The slips were the visible damage. The underslips were the sneakier kind, where water undermines the road from below. The surface can look normal, or show only cracks along the seal, while the ground carrying it has given way underneath.

The road was closed completely for two weeks. To reopen even one lane, contractors drove steel pipes down through the carriageway into solid ground beneath the moving material, so that the lane past the main slip sits on piles rather than on creeping earth. Newswire covered the reopening at the time.

The months since have been a slow climb back, repeatedly interrupted.

• Mid April — the storm closes the gorge completely, with about 39 slips and six underslips
• 2 May — the road reopens from 5pm under escorted convoys, at set times only, and stays shut during weekday daytime hours while crews work
• 22 May — convoys end and the gorge moves to single lane stop and go past the main slip
• 25 June — closed overnight after fresh movement on the slip; a protective barrier catches the debris
• 25 July — a new slip of around 500 cubic metres closes the road again; one lane reopens the following evening
• 22 to 25 August — the planned four day closure to install the culverts

The July slip was the closest call of the year. It came down just after 5am, minutes after the magnitude 5.9 Taumarunui earthquake struck at 4.51am, in front of a family driving through in steady rain. Nikita Phillips told 1News her partner saw it first. “Oh my god, hun, look, the cliff’s coming down,” he said. A road worker sprinted toward their truck yelling “go, go, go” and the family reversed clear. No formal connection between the earthquake and the slip has been established, but the timing gives a sense of how finely balanced the slopes now are.

Why the Awakino Gorge keeps slipping

The short answer is that the Awakino Gorge is cut through some of the weakest rock a state highway in New Zealand has to cross, the rock gets weaker when it is wet, and the road runs along the bottom of steep slopes with a river eating at their base.

The hills either side of the road are mostly Mahoenui Group mudstone, laid down on the sea floor in the early Miocene, roughly 19 to 22 million years ago, and known across the King Country as papa. The Geoscience Society of New Zealand’s field guide to the region describes thick beds of this blue-grey mudstone underlying the whole district. Papa behaves less like rock and more like very stiff clay. Exposed to the weather it dries, cracks and flakes, and when it soaks it softens.

Water is the trigger. Rain soaking into a slope fills the pore spaces in the rock and pushes outward, and that pressure lowers the friction that holds the slope together. This is why the recovery work is so focused on drainage, and why the August culverts matter more than their size suggests. Every drain and culvert exists to bleed water pressure out of the hillside.

The timing of the water matters too. An intense storm strips loose material off the surface in shallow slips, which is most of what came down in April. But the deeper, slower movement in a papa slope answers to the wetness of the whole season, as groundwater rises over weeks. That is why the main slip kept creeping after the skies cleared, and why the gorge can close on a geotechnical call in perfectly fine weather.

The river finishes the picture. The Awakino River runs hard against the base of the slopes and keeps cutting into them, and the highway itself was benched into the same toe of the hills a century ago. Between the two, the bottom of the slope, the part that buttresses everything above it, is permanently being nibbled away.

A century of workarounds

None of this is new. The highway through the gorge has always been a negotiation with the landscape, and the history of the wider route is a history of building around its worst sections rather than fixing them in place.

• 1920s — the single lane Awakino Tunnel is cut, and remains a pinch point on the highway for the next hundred years
• Mid 2010s — the Government’s Accelerated Regional Roading Package funds investigations along the 58 kilometre Awakino Gorge to Mt Messenger corridor, which NZTA describes as windy, narrow and prone to rockfalls and slips
• 2019 to 2021 — the Awakino Tunnel Bypass replaces the old tunnel with a 2.3 kilometre realignment and two new bridges over the Awakino River, involving roughly 200,000 cubic metres of earthworks; traffic switches onto it in June 2021
• 2022 — construction starts on Te Ara o Te Ata, the six kilometre Mt Messenger bypass at the southern end of the corridor, with its 235 metre tunnel and two bridges
• Late 2025 — the Mt Messenger tunnel breakthrough is reached; the bypass remains under construction and is not yet open to traffic

The pattern is easy to see. Where this stretch of State Highway 3 has been permanently improved, the fix has been to abandon the worst piece of road entirely and build a new one around it. The slip-prone heart of the Awakino Gorge has never had that treatment. It gets repaired in place, storm after storm, on the same narrow bench between the papa slopes and the river.

What every closure costs

SH3 is the main artery between Taranaki and the Waikato, and there is no quick way around it. Every closure sends freight on an inland detour that adds hours each way, and sends Taranaki patients the long way round to specialist appointments. New Plymouth District Council counted 13 weather-related closures on the route between Mt Messenger and SH4 from July 2021 to May 2026, a tally the June and July slips have since added to.

The sharpest pain lands on the small towns that live off passing traffic, above all Mōkau, just north of the gorge. Dev Rawat, who owns the River Run Cafe, told RNZ the closures were devastating. “We depend on the people who passing through State Highway 3 and when the road is shut no one come into the cafe and that impact us lots,” he said. Butcher Bryan Lester told RNZ he served five customers in a full day during the last closure, and says he will likely lock the door for the four days in August.

New Plymouth mayor Max Brough has been blunt about the trade-off. “Look, it’s disappointing, but you can’t fix something without doing the work, can you?” he told RNZ. The disruption is the price of the repair, but the towns along the road are paying it again and again this year.

What the $50 million will pay for

In May, Budget 2026 set aside $50 million for the Awakino Gorge, carved out of a $400 million package for strengthening vulnerable parts of the state highway network. The programme is expected to include slope stabilisation, improved drainage and culverts, small retaining walls, and works to slow the river’s erosion of the road corridor.

Each item on that list answers one of the failure mechanisms in this article. Stabilisation and retaining walls hold the papa slopes; drainage bleeds off the water pressure that weakens them; river protection stops the Awakino undercutting the toe. It is the first funded attempt to treat the gorge’s problems as a system rather than as a series of emergencies.

What the $50 million is not is the work happening this month. NZTA describes the August culverts as part of the recovery from the April storm, aimed at getting the road back to two lanes. The resilience programme is the longer game, and it is still being designed, with Taranaki’s councils among those with a say in how the money is spent. Nobody in Mōkau should expect to see it at work this winter.

What happens next

If the culverts go in on schedule, the water coming off the main slip will finally have somewhere to go other than the road, and NZTA can push on toward restoring two lanes through the gorge. The monitoring will not be going anywhere. The slip is watched around the clock by cameras and sensors under a trigger action response plan, and if movement passes set thresholds the road will close again at short notice, in any weather.

Anyone travelling between Taranaki and the Waikato from 22 August should plan for the SH4 and SH1 detour, keep an eye on the backup dates in case weather shifts the closure, and check the NZTA Journey Planner before setting out.

Have the Awakino Gorge closures affected your travel or your business? Tell us in the comments below.

This article was written by AI, briefed to report the facts, hopefully without some of the bias people bring to the job 🙂