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Fireworks set off next to a pony club scared a horse called Kracker so much it ran into an object, severely hurting itself and had to be put down.
The blast was set off a week before fireworks started selling in shops, which worries the SPCA.
The parents of the 14-year-old girl who owned the show-jumping, bay-coloured mare were yet to tell her what had happened as of last night.
She had just competed in an event on Saturday at Woodhill Forest with Kracker, who she had owned for about 10 months, and had planned on entering several more competitions this year.
"The horse was someone's love and their sport project," club district commissioner Geoff Wood said.
"Hours of work training the horse for the competitive season went up in a flash, bang and a puff of smoke.
"She basically had everything planned out for the year and it's all gone down the drain."
It is understood neighbours called police after midnight on Monday after hearing fireworks near the Te Atatu Pony Club which sounded like "cannons going off".
Mr Wood said he thought the offenders had put the fireworks inside containers to increase the sound and then let them off.
Members of the club noticed the 14-year-old horse, which had head and shoulder wounds, lying in one of their enclosed paddocks with 23 other horses, when they came in on Monday morning.
"Like most animals when frightened, they have a tendency to run and it's obviously got such a fright it ran into a solid object," Mr Wood said.
"The family, club and fellow equestrian riders are saddened that the action of a few has led to this tragedy. It understood that the horse had seven fractures and obviously suffered throughout the night until it was discovered the next morning."
The horse was taken to the Redvale Park Equine Hospital in Dairy Flat but had to be put down yesterday morning.