Dozens of horses were injured and at least three were destroyed during and after an electrical storm last weekend, described by some Northlanders as the fiercest they had ever experienced.

In a grim weekend for horse lovers, a mare also had to be put down after it was savaged by dogs in the Bay of Islands on Sunday afternoon.

According to the MetService, Saturday night's storm sparked 13,000 lightning strikes in the 12 hours to 8am on Sunday, with 841 of those on land. Most were in Northland, but Auckland, Coromandel and Great Barrier were also affected.

The storm caused surprisingly little damage to infrastructure, but the animal toll was high, with on-call vets rushed off their feet. Garth Riddle, of Bay of Islands Vets, said he had put one horse down at Haruru Falls due to injuries suffered when it crashed into a fence, and treated at least 10 others just in the Paihia-Waipapa area on Sunday.

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Two more had to be put down at Kokopu, near Whang─ürei.

Mr Riddle was still tending to injured horses and calves on Monday, and had heard reports of dead deer.

"But when you look at the numbers that have run through fences and smashed things, it's quite astounding how few have really bad injuries," he said, although he had never before treated so many animals as a result of a single storm.

One of the lucky ones was Molly, a 14-year-old mare owned by Ingrid Wilkinson and her daughter Sophie, in Waipapa. They were woken at 2.30am by the fiercest storm they had ever seen, with thunder and lightning directly overhead.

When they went to check on their horses at 6am they discovered the fence around Molly's paddock had been flattened, along with another along a deep stream. They eventually found Molly 200m downstream, where she had found a spot shallow enough to stand. By the time they managed to get her out, after five hours in the water, she was shaking and barely able to walk.

"Any longer and she would've drowned," Mrs Wilkinson said.

Remarkably, her only injuries were a gash to her nose and cuts to her legs.

Mr Riddle said he had treated most of the injured animals with tetanus shots, antibiotics and anti-inflammatories, adding that there was little that horse owners could do even if they knew a storm was coming.

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"It's a very unfortunate thing. They get scared and run. It's just their nature," he said. And a horse could get spooked in a stall as easily as it could outside.

Meanwhile members of the Paihia Pony Club were in shock after a 6-year-old thoroughbred mare was mauled by dogs and had to be put down on Sunday.

The owner was devastated, and did not want to be named, her distress compounded by incorrect information spread via social media.

The attack is thought to have occurred between 3pm-4pm on Sunday, but was not witnessed. The mare, which was being trained as a show hack, had been checked after the storm and was fine earlier that afternoon. With a severe wound to one knee and lesser injuries to her other legs, she was discovered by a teenage club member who was checking on another horse.

The more was put down and buried that afternoon.

Jeremy Kirwan, acting environmental services manager at the Far North District Council, described the attack as "particularly worrying". Two animal management officers went door-to-door in Haruru Falls on Monday to gather information.

"I share the concerns of residents about this attack, and urge anyone who can help us trace the dogs responsible to contact us immediately," Mr Kirwan said. "This attack again underlines our message that all owners must ensure their dogs are under control at all times."

In July a series of attacks on pets in Haruru Falls culminated in the deaths of 12 prized chickens. Council staff later seized a dog from a Yorke Rd address, the second from the same property.