SH1 Mangamuka Gorge slip repairs

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Located in Te Tai Tokerau Northland, the Mangamuka Gorge is an essential route for both locals and visitors. However, the gorge has faced repeated closures due to severe weather-related damage. The most recent closure began in August 2022 following a major weather event that caused 15 significant slips, six of which were deemed critical, and almost all required complex engineering solutions.

Further weather events, combined with a record amount of rainfall in 2023, led to additional slips, added complications to existing slip sites, and increased the scope of work. The total number of slips more than doubled to 36, with 16 deemed critical. This subsequently resulted in the initial works programme being extended.

The team has worked at pace to restore this vital connection, overcoming challenging geology and difficult terrain. The repair effort has been a massive undertaking, involving extensive engineering work, including reinforced concrete piles anchored deep into the maunga (mountain), improved drainage systems, and environmental considerations to protect local wildlife.

The project has worked closely with local hapū from the outset, with representatives and workers being a key part of the project and delivery teams. At its peak, approximately 75% of the workforce and many of the suppliers were from the local community, underscoring the project’s significant connection to the region.

Set to reopen on Friday 20 December 2024, the road will reconnect whānau, friends, businesses and communities, ending long detours and providing essential access to services. The reopening brings significant relief to the communities impacted by the closure, restoring this critical economic and social lifeline with a safe and reliable state highway network.

• August 2022: 15 significant slips (6 being critical) resulted in State Highway 1 (SH1) through Mangamuka Gorge closing.
• August – November 2022: geotechnical investigations were carried out so we could understand the full extent of the damage and ground conditions within the gorge.
• December 2022: an initial $100m of funding was secured to restore the state highway to its original condition.
• January – late March 2023: investigations, planning and design work was refined to further understand the damage and how best to repair this challenging transport corridor.
• 20 February 2023: the project was awarded to the main contractor and the project kicked off.
• April 2023: additional weather events resulted in a total 35 slips, of which 15 are deemed critical, and require complex engineering solutions.
• 19 June 2024: a new critical slip formed, which took the total number of slip sites to 36 – 16 of these were deemed critical.
• June 2024: further Crown funding of $65m was approved to address the additional slips.
• November 2024: a final $37m from the National Land Transport Fund (NLTF) was approved to address cost escalations caused by poor ground conditions, high number of rain days and additional scope associated with further ground movement in 2024.*
• February 2023 – December 2024: slip repair work progressed at pace, with the road reopening to traffic at 12:01am on Friday 20 December 2024.
• January 2025 – April 2025: Finishing works to complete the repairs, including final anchor work on the remaining two critical slips. The road will remain open during this period with planned stop/go temporary traffic management and reduced speeds in place.

*Note: the costs are rounded figures. The estimated total cost is $204 million.