Huge quantities of sand have disappeared from the protective barrier sandspit that critically protects the Northland township of Mangawhai.
New Zealand’s first in-depth aerial sandspit survey has revealed more than 420,000 tonnes of non-replenishing sand have been lost from Mangawhai sandspit.
An estimated equivalent of 26,000 truckloads of sand has been lost in six years according to the high-resolution drone survey.
The latest research showed the spit’s major large southern dune had moved west by 10 to 25 metres and decreased in elevation. There had also been harbourside erosion up to 15 metres in places.
“It confirms loss of significant volume of sand from the spit,” said University of Auckland coastal geomorphologist Professor Dr Mark Dickson.
Along with Research lead coastal geomorphologist and former University of Auckland professor Dr Terry Hume, Dickson presented the new research findings for the first time to a meeting of Northland Regional Council (NRC), Department of Conservation, Kaipara District Council (KDC), Environs Te Uri o Hau, Mangawhai Matters community group, MHRS members and residents in Mangawhai on Friday.
Mangawhai Matters committee member, former Massey University professor of resource and environmental planning Dr Phil McDermott said the new work helped bring the sustainable management of the harbour and spit to life.
Mangawhai sandspit is one of a tiny handful of “drumstick” spits around New Zealand estuaries. It is shaped like a chicken drumstick making it more vulnerable to erosion and breaching with the sea washing right over it at its narrowest waist-like point. The sandspit is even more vulnerable because its covering vegetation was lost through fires 800 years ago.
The sandspit was surveyed in early 2025 high resolution drone LIDAR (light detection and ranging) surveying by university researchers for the Mangawhai Harbour Restoration Society (MHRS). This built on less detailed August 2024 LIDAR aerial surveying from a plane.
MHRS chairperson Peter Wethey said the new March research results brought the first concrete, scientific evidence to underpin managing and protecting the critical landform feature into the future. It would help provide direction for the community’s efforts.
The society was at the forefront of the 1991 Big Dig to reopen Mangawhai Harbour’s northern entrance that closed when the spit was overtopped, as well as to close a breach in the sandspit “waist” about a kilometre south after a huge 1978 storm. This was exacerbated by Cyclone Bola in 1988.
Hume said there with a high risk of another breach.
“Our latest research confirms the prospect of another breach within 50 years is high,” Hume said.
His research suggested the greatest potential for the sea breaching the spit was at the spit’s “waist” where it is currently about 440m wide.
Hume said urgent work was needed to work out which newly identified erosion and inundation hot spots to most focus mitigation efforts into. It was necessary to identify and plug gaps in dunes and pathways where the sea might flood in.
The quantity of sand needed to fill these needed to be calculated, along with whether the sand was coming from harbour dredgings, dunes or the beach. Equipment to do this was needed. Consents were needed and the cost of the mahi worked out.
Active management was needed to review options. Ongoing monitoring of the spit’s shorelines, topography and volume was also necessary.
Hume said sand replenishment work in vulnerable areas, sand trap fencing and planting more than 100,000 dune plants by the society and community have been involved with over three decades has been identified by researchers as an effective and durable solution to building spit resilience.
The 800m long man-made bund wall along the spit’s inner shoreline was playing a key protection role. This was built stretched north from the spit’s 1978 southern breach to stop sand from blowing into the harbour and closing.
Local Democracy Reporting is local-body journalism co-funded by RNZ and NZ On Air.
