One person who was at the scene said it was understood the man had a broken leg and a head laceration.
Police said two horses died at the scene and one was euthanised by a vet called when the animal was found injured after bolting from the scene.
Another, which had also bolted, was soon found uninjured and put in a paddock belonging to a resident who checked the horse after dawn, and fed the animal carrots.
Animal control staff from the Hastings District Council removed the dead horses, and the carcasses were claimed by the owner later in the day, said council communications manager Diane Joyce.
Early-morning road user Lisa Mullins was concerned about the amount of time it had taken animal control to call a vet and for the vet to arrive to attend the horse that had to be euthanised.
She said in Facebook posts she was on her way to work, and stopped at the scene, going with a police officer to where the injured horse was standing - "alone, no halter, no nothing, unsteady and afraid".
She said it was about 4.30am and she stood with the horse to make sure he "wasn't going anywhere and to try to keep him calm".
"Animal control came quite some time later and I asked when was the vet coming," she wrote.
It was apparently only then that a vet was called, and an hour and a half before one arrived and euthanised the animal.
"I stayed while he was put to sleep," she wrote. "Was a very sad start to the day for all concerned."
Police said last night there had been no lighting in the area of the crash, and that police were trying to establish why the horses were on the road.
"At any time of the year drivers need to be vigilant when driving on rural roads as escaped or wandering stock is always a possibility and hazard."