Three dogs have died from eating toxic karaka tree berries this week, prompting a warning from a senior vet.

Veterinary Specialist Group's Dr Mark Robson said they had seen three dogs poisoned by the berries this week.

"It's the time of year when the berries are at their ripest which means they are highly attractive to dogs".

And this year appeared to be "particularly bad for toxicity", Robson said.

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"The berries ripen and fall at this time of year, and the toxin levels vary for reasons we don't understand.

"We don't know why dogs like the berries, but dogs are scavengers and will eat anything."

Owners should not let dogs off the lead to roam near karaka trees and they were urged to seek the advice of a vet immediately if they suspected their dog had eaten the berries.

"Get to a vet clinic as fast as you possibly can. Your dog may show signs of nausea and could have diarrhoea and restlessness.

"This quickly progresses to more severe gastrointestinal signs and neurologic signs that include pacing, limb rigidity, crying, seizures then unconsciousness."

Chance of survival was lower once a dog started to have seizures and became unconscious.

Serious cases required specialist 24-hour care, he said.

The lower branches of a Karaka tree. Photo / Supplied
The lower branches of a Karaka tree. Photo / Supplied