Cape Rēinga lighthouse slip due to poor workmanship, DoC learns in report

The slip to the east of the lighthouse at Cape Rēinga/ Te Rerenga Wairua occurred after fill material was paved over, a report to DoC says.

A slip which threatened the Cape Rēinga lighthouse last year was likely due to fill material being poorly placed in 2003, according to a report to the Department of Conservation.

The lighthouse and wider Te Paki Recreation Reserve were impacted by Northland’s extreme high rainfall in early 2023, including a May storm where more than 20mm of rain per hour fell at the top of the country.

Cape Rēinga/Te Rerenga Wairua - a place of great significance to Māori - suffered a major slip to the east of the lighthouse and had to be closed while it was fenced off. Access was later restricted while repairs were carried out.

The rest of Te Paki Coastal Track, once shortlisted for a New Zealand Great Walk, was also closed by slips.

The eastern part remains closed indefinitely while DoC and Ngāti Kurī work on improvements.

An investigation into the damage by outdoors specialists Frame Group for DoC was obtained by the Northern Advocate under the Official Information Act.

It found the slip at the lighthouse was most likely caused by water getting into the fill material that was poorly placed during work in 2003, followed by a series of DoC decisions which made the situation worse.

The 2003 landscaping work involved moving a rock mound on the western side of the lighthouse and using it as non-engineered fill to raise a low area to the east of the lighthouse.

In 2005, the area was paved and a rock wall was installed to protect visitors from falling down the steep slope. DoC staff decided to extend the paved area over the fill to incorporate a garden and lengthened the rock wall greater than originally designed, the report said.

In 2013, cracking appeared in the paved area over the fill and the sealed access track. A report by geotechnical engineers Tonkin and Taylor attributed the cracking to the non-engineered fill settling on top of sand deposits which cover the basalt base rock.

A 2014 report by Frame Group recommended a range of options from doing minimal crack sealing to installing extensive retaining walls. DoC decided to do crack repair only, on the basis that catastrophic failure was unlikely.

The platform to the east side of the lighthouse at the Cape Reinga/Te Rerenga Wairua collapsed in 2023 and site access was restricted while repairs were carried out.

Frame Group found the severe rain in May 2023 saturated the non-engineered fill and underlying sand, with the garden area providing a permeable entry point for the rainwater.

Surface stormwater also flowed down the sealed access track into the lighthouse area, entering open cracks in the paving and scouring the slip path as it flowed over the edge of the paving.

Frame Group recommended retreating the paved area and wall back from the steep edge, removing the garden, improving drainage on the access and stabilising the slip.

Meanwhile, the difficulties of opening access in another part of Te Paki track have been highlighted in the same report by Frame Group, which found the Waitahora Lagoon Boardwalk is now frequently covered with water.

The 270m boardwalk was designed in 2011 to provide walking access between the now closed Whangakea/Pandora Campsite and Kapowairua (Spirits Bay), instead of a potentially hazardous mouth crossing of the Waitahora Lagoon.

Work started in 2011 in what was meant to be a stable part of the lagoon but the boardwalk was flooded by 500mm of water before work even finished, due to the lagoon’s entrance being blocked.

the Waitahora Lagoon Boardwalk is now frequently covered with water, such as during this inspection shortly after the May 2023 storm.

The freshly installed piles were lifted out of the sandy lagoon bottom by the floodwaters and extra anchors needed to be used before the boardwalk was finished in 2012.

However, the flooding has become more frequent and, during the cyclones in early 2023 including Cyclone Gabrielle, the boardwalk was covered by 2m of water.

The water levels fell enough to see at least 10m of the boardwalk has been washed away but, when Frame Group went to check on the boardwalk, it was again flooded by 500mm of water, with the lagoon entrance again blocked.

Frequent and prolonged flooding may become the norm as the effect of climate change deepens, Frame Group said.

It recommended DoC get specialist coastal engineering advice on the lagoon’s likely future dynamics, and consider crossing the wetland further inland.

Denise Piper is a news reporter for the Northern Advocate, focusing on health and business. She has more than 20 years in journalism and is passionate about covering stories that make a difference.