Toxic Plants & Algae for Dogs in New Zealand: Karaka Berries, Tutu, Toxic Algae

Every summer and early autumn, New Zealand vet clinics see a predictable spike in two very different kinds of poisoning case: dogs that have eaten fallen karaka berries on a walk, and dogs that have licked, eaten or swum through the dark, slimy mats of toxic algae that bloom in rivers and lakes over the warmer months. Neither hazard is rare or exotic — both are common, native to the New Zealand outdoors, and both can kill a dog within hours if the exposure is missed or the warning signs are mistaken for something else.

This guide covers the native plants that most often poison New Zealand dogs — karaka, tutu, ngaio and rangiora — and, separately, the toxic-algae (cyanobacteria) risk in rivers and lakes nationwide. The two hazards have different causes and different seasons, but they share the same hard truth: there is no antidote for either. Prevention and fast emergency care are everything.

Karaka berries: a delayed-onset neurotoxin hiding in plain sight

The karaka tree (Corynocarpus laevigatus) is one of New Zealand's most familiar native trees — planted in parks, along walkways, on marae and in coastal reserves throughout the North Island and northern South Island. Its orange-yellow fruit ripens and falls between January and April, and it is this fallen fruit — specifically the hard kernel inside, not the fleshy outer covering — that poses the danger to dogs. The kernel contains karakin, a neurotoxic glycoside that acts on the central nervous system.

Māori traditionally prepared karaka kernels for human consumption through an extensive process of roasting, soaking and fermenting over several days to break down the karakin — a labour-intensive tradition that underlines just how toxic the raw kernel is. That preparation does nothing to help a dog: a dog that finds and swallows fallen berries or discarded, unprocessed kernels on a walk gets none of that detoxification, only the raw neurotoxin.

• Early signs: vomiting, diarrhoea, reduced appetite, abdominal pain, general weakness and lethargy.
• Progressing signs: loss of balance/coordination, paralysis of the hind legs, confusion.
• Severe signs: convulsions and seizures, reduced or laboured breathing, respiratory failure — this stage can be fatal.

There is no specific antidote for karakin poisoning. Veterinary treatment is supportive: inducing vomiting if the ingestion is recent and the dog is presented early enough, activated charcoal to limit further absorption, intravenous fluids, and anticonvulsant medication if seizures occur — all decisions made and administered by a vet, never at home. The New Zealand Veterinary Association has specifically warned dog owners to seek emergency veterinary care immediately if a dog is suspected of eating karaka berries, precisely because of the delayed and potentially severe symptom pattern.

Tutu, ngaio and rangiora: the other native plants NZ dog owners should know

Karaka gets the most attention because of its seasonal spike in vet cases, but it isn't the only native plant that poses a genuine risk. Tutu and ngaio in particular are well documented in New Zealand toxicology as capable of killing animals, including dogs, that ingest them.

• Tutu (Coriaria arborea) — widely regarded as New Zealand's most potent native plant toxin. Nearly every part of the plant (except the fleshy petals of the fruit) contains tutin, a poison that acts directly on the central nervous system. Signs include drooling, agitation, convulsions and death from respiratory paralysis in severe cases. Tutu and ongaonga (tree nettle) are the only native New Zealand plants documented to have killed people historically, underlining the potency involved — though dogs encounter and ingest it less often than grazing livestock do.
• Ngaio (Myoporum laetum) — a common coastal native tree whose leaves and reddish-purple berries contain ngaione, a liver toxin. Ingestion can cause severe photosensitivity (a sunburn-like skin reaction), liver damage and, in serious cases, liver failure and death — documented in both dogs and livestock.
• Rangiora (Brachyglottis repanda) — listed among New Zealand's poisonous native plants in general toxicology references, though dog-specific case documentation is thinner than for karaka, tutu or ngaio. Treat rangiora with the same general caution as any plant on the poisonous-native-plant list: discourage chewing, and contact your vet if your dog ingests a significant amount of any unfamiliar plant, whether or not it is specifically documented as dangerous to dogs.

Toxic algae (cyanobacteria): a completely different, equally deadly risk

Toxic algae is not a plant hazard at all — it's a bacterial one, and it belongs in this guide because it is one of the most searched, most serious, and most misunderstood outdoor risks facing New Zealand dogs, especially in summer. Benthic cyanobacteria (mat-forming blue-green algae, historically called Phormidium and now more often classified under Microcoleus) grow on the beds of rivers and lakes throughout the country. When conditions are right — typically warm, settled weather with low river flows over summer — these cyanobacteria form thick mats on rocks and riverbeds that can detach and wash up on the banks, or float free in the water.

The toxins these cyanobacteria produce (a family of compounds that include neurotoxins and hepatotoxins) are extremely potent. Regional council and Land, Air, Water Aotearoa (LAWA) public-warning material has repeatedly cautioned that an amount of toxic algae roughly the size of a 50-cent coin can be enough to kill a dog — a figure used in public-safety messaging rather than a precise toxicological threshold, and one that will vary with the toxin concentration in a given bloom. Dogs are considered more vulnerable to this hazard than people specifically because of their behaviour: they wade, dig, sniff and scavenge directly at the water's edge and on riverbeds where the mats accumulate, in a way most people never do.

• Lethargy and general weakness, often the first sign noticed
• Muscle tremors and twitching
• Rapid or laboured breathing
• Loss of coordination progressing to paralysis
• Convulsions/seizures
• In the most severe cases, collapse and death — sometimes with very little warning before collapse

What to do if you suspect poisoning — and what not to do

• 1. Treat it as an emergency immediately — do not wait to see if symptoms appear, especially with karaka, where signs can take up to 48 hours to show.
• 2. Call your vet or the nearest 24-hour emergency clinic before you leave, if possible, so the team can prepare.
• 3. Do NOT induce vomiting at home unless a vet specifically instructs you to and tells you how — inducing vomiting incorrectly, or when a dog is already showing neurological signs (poor coordination, seizuring), can cause aspiration and make things worse.
• 4. Bring evidence if you can — a photo of the plant/berries/algae mat, or a sample in a sealed bag/container (handled carefully, and kept away from other pets and children) — this helps the vet identify the toxin and tailor treatment.
• 5. Note the timeline — when you think the exposure happened, roughly how much was involved, and when symptoms (if any) first appeared. This detail materially helps emergency treatment decisions.
• 6. For toxic algae specifically, rinse your dog's coat and paws with clean water as soon as possible if they have been in contact with a mat or contaminated water, to reduce the chance of further ingestion from licking their fur — but this is in addition to, not instead of, urgent veterinary care.

Prevention, by season and region

Both hazards are largely preventable with a bit of situational awareness — and both follow reasonably predictable seasonal and regional patterns that New Zealand dog owners can plan around.

• Learn to identify karaka trees in your local parks and walking routes, and keep dogs on a lead or under close control near them during Jan–Apr, especially where fallen fruit is visible on the ground.
• Check your regional council's water-quality or LAWA alerts before visiting a river or lake over summer — Environment Canterbury, Greater Wellington, Environment Southland and others publish current toxic-algae warnings by specific river/site.
• Look before you let your dog swim — dark brown/black slimy mats on rocks or the riverbed, light brown/white dried crusts on the bank, or vivid green scum on the water surface are all signs to stay well clear, at any site, warning or no warning.
• When in doubt over summer, choose known, monitored swimming spots over unfamiliar rivers, and avoid slow-moving, warm, low-flow stretches where cyanobacteria mats are most likely to form.
• Keep dogs away from decaying vegetation and unfamiliar berries generally, not just karaka — a "leave it" cue and close lead control during peak native-plant fruiting season is the most reliable prevention tool an owner has.