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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 MetService, Saturday night's storm brought 13,000 lightning strikes in the 12 hours to 8am on Sunday, 841 of which were 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 and on-call vets were rushed off their feet.
Garth Riddle, of Bay of Islands Vets, said he had put one horse down at Haruru Falls because of injuries from a fence and treated at least 10 others just in the Paihia-Waipapa area on Sunday.
Another two spooked horses had to be put down in Kokopu, near Whangārei.
Yesterday, Riddle was still tending to injured horses and calves 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.''
Riddle said he'd 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'd experienced, with thunder and lightning directly overhead.
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,'' Wilkinson said.
Remarkably, her only injuries were a gash to her nose and cuts to her legs.
Riddle said he had treated most of the injured animals with tetanus shots, antibiotics and anti-inflammatories.
There was little horse owners could do even if they knew a storm was coming.
''It's a very unfortunate thing, they get scared and run. It's just their nature.''
A horse could get spooked in a stall as easily as it could outside, Riddle said.
Meanwhile, members of Paihia Pony Club, on Haruru Falls Rd, are in shock after a 6-year-old thoroughbred mare was mauled by dogs and had to be put down on Sunday.
The owner is devastated and did not want to be named, her distress compounded by a social media furore spreading incorrect information.
The attack is thought to have occurred between 3pm and 4pm on Sunday but was not witnessed. The adult mare, which was being trained as a show hack, had been checked after the storm and was fine earlier that afternoon.
The horse had suffered a severe wound to one knee and lesser injuries to her other legs. The gruesome injury was discovered by a teenage club member checking on another horse. The horse was put down and buried that afternoon.
Jeremy Kirwan, acting environmental services manager at the Far North District Council, described Sunday's attack as "particularly worrying".
Two animal management officers went door-to-door in Haruru Falls yesterday to gather information they hoped would identify the offending dog or dogs.
"I share the concerns of residents about this attack and urge anyone who can help us trace the dogs responsible to contact us immediately. This attack again underlines our message that all owners must ensure their dogs are under control at all times."
If the council could obtain compelling evidence the owners would be prosecuted and the dogs destroyed.
More lightning was forecast for Northland last night.
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.