• Sea spurge is an invasive plant that could take over large areas of New Zealand's coast
• Its sap is toxic and can cause temporary blindness
• A newly discovered infestation at Mitimiti, in the Far North, is the biggest to date

A toxic plant, which can temporarily cause blindness and is threatening to take over large areas of New Zealand's coast, has been found in large numbers established on a remote Northland beach, and efforts are now on to get rid of it before it is too late.

Sea spurge, which is native to the Mediterranean, first arrived in Aotearoa in 2012 on ocean currents from Australia, where the invasive plant has already caused environmental havoc.

Since 2020 it has been found at more than 30 locations along the west coast of the North Island, mostly between Taranaki and Wellington.

However, a new find at Mitimiti, in a remote part of the Far North's west coast, has dwarfed all previous infestations and set alarm bells ringing.

Now, Far North hapū and biosecurity officials hope to eradicate what they say is the largest infestation to date.

It consists of more than 200 adult plants spread over two sites, totalling more than 2300sq m.

Mitimiti hau kāinga representative Debbie Martin said a local kaitiaki [guardian] made the "incredibly concerning" discovery in an area known as Moetangi, in early October.

Local hapū Te Tao Maui and Te Hokokeha had been working hard to replant the dunes with native tihetihe, or spinifex, and pīngao - and had earmarked Moetangi as the next area for planting.

"Unfortunately, what sea spurge does - and the evidence is pretty glaring from Australia - is that it completely sweeps over native plants. It changes the natural pattern of sand and it displaces any bird life. What we would see if it was left unchecked would be a blanket of sea spurge replacing our native tihetihe and pingao, which our taonga species toheroa depends on," Martin said.

Sea spurge seedlings emerging from the beach at Mitimiti, in Northland. Photo: Supplied

Given their isolation and long tradition of looking after the coast, local hapū were taking the lead on decision-making and managing sea spurge, while making use of the resources of the Northland Regional Council (NRC), the Department of Conservation and Ministry for Primary Industries.

"We've made a really considered decision based on all the available information, including our own matauranga, as to what pathway we're going to take for eradication. Part of the removal process will include herbicide treatment, and this will enable us to get really quickly on top of this infestation, which is the largest in Aotearoa."

Plants would also be pulled out manually, Martin said.

Herbicides would be used for targeted spot treatment only, and precautions would be taken to ensure protection of people and the environment, including water and kai moana testing.

"We really want to get it before the summer rush of our whānau coming home. The plant is toxic, so if you break it, the sap can burn your skin and also cause temporary blindness. The area where the sea spurge is growing is where our beautiful awa [river] Moetangi meets the sea, so it's where kids often swim and play in summer."

The infestation site was currently fenced off, and was likely to remain that way all summer.

Martin said the seeds were thought to travel on ocean currents from Australia, so monitoring for fresh infestations would likely have to continue "forever and ever".

NRC biosecurity manager Joanna Barr said previous sea spurge sites in Northland were small, so the plants could be removed immediately.

When the Mitimiti infestation was discovered, hau kāinga and DOC staff held a working bee to map it and "deadhead" adult plants to prevent any further seeding.

Experience from elsewhere in New Zealand was that sea spurge could be eliminated through consistent control, Barr said.

Sea spurge Photo: Northland Regional Council/ Frank Vincentz

"We expect new sites to be found in future, because sea spurge seed can float across on ocean currents from Australia. The key is finding them early, so anyone who thinks they've seen sea spurge shouldn't touch it, and call the MPI hotline to report it so it can be controlled."

Barr said sea spurge was first detected in Northland late last year on Poutō Peninsula, on the northern side of the Kaipara Harbour.

It had now been found at 11 sites around the region, with three near Mahuta Gap on the Poutō Peninsula, five in the Waipapakauri area on Te Oneroa-a-Tōhe [Ninety Mile Beach], and a single plant near Ahipara. The two sites at Mitimiti were about 200m apart.

According to the Ministry for Primary Industries (MPI), sea spurge posed a serious threat to New Zealand's coastal environment.

Left uncontrolled, it could spread quickly and dominate beaches and dunes, taking over areas where native plants grew and destroying habitat for birds.

Its sap was toxic to humans and animals, and could cause skin irritation, as well as temporary blindness. If it spread into pasture it could also affect the health of livestock.

MPI said Biosecurity New Zealand, DOC, regional councils, iwi and hapū, and local groups were working to stop the pest plant becoming established in New Zealand.

Outside Northland, sea spurge had also been found at Karekare Beach and Aotea Harbour in the Auckland Region; Mokau in south Waikato; Ōakura in Taranaki; Scotts Ferry, Himatangi, Foxton Beach, Waitārere and Ōhau in south Horowhenua-Manawatū; and Waikawa Beach and Paraparaumu in on the Kāpiti Coast.

A fungal biocontrol agent is being trialled for use against sea spurge in Australia, but would require extensive testing in New Zealand to ensure it did not harm native plants.

How to recognise sea spurge:

• Sea spurge (Euphorbia paralias) looks like a small shrub, usually up to half a metre tall.
• It has multiple stems that are often reddish at the base. Its spiky, tightly packed bluey-green leaves are 4-20mm long and 1-16mm wide, with greenish flowers on the end of the stems.
• It looks similar to the native spurge, waiūatua/waiū-o-kahukura (Euphorbia glauca) and New Zealand linen flax (Linum monogynum). However, the native spurge has much larger leaves: 30-80mm long.
• Sea spurge is usually found on dunes or among debris, washed up on beaches.

What to do if you find sea spurge:

• Don't disturb the plants, because that can spread the seeds. The sap is toxic so don't cut or break the stems.
• Take clear photos of the location and the plants, ideally including closeups of parts of the plant, especially the flowers.
• Record your location accurately as possible, using Google maps or GPS if possible.
• Call the MPI hotline on 0800 80 99 66, or report it online at https://report.mpi.govt.nz/pest/

Photo: Ministry of Primary Industries

This story was originally published by RNZ