An Epsom doctor among the farm and sculpture park's visitors helped her stay calm and sit upright until an ambulance arrived.
Although Hart could see humour in the incident, she said it could have been far different if she hadn't worn heavy tramping pants. The thick material prevented the horn from piercing her groin, she said.
"I thought it was funny - if anyone is going to get tossed by a yak it is going to be me. But the next day I thought, 'My God, I'm lucky to be alive.' If it was a kid it would have been dead because it would have been at head height."
She said an apologetic farm manager brought flowers to the hospital last Sunday. He told her the yaks had been moved to a wired enclosure and the Ministry of Business, Innovation and Employment was investigating.
Noel Lane, Gibbs' son-in-law and a director at the farm, said it was the first time a yak had attacked a visitor since they were introduced 20 years ago. "We have absolutely no idea why it happened but either way it's completely unacceptable."
The yaks would no longer be allowed in the same paddocks as visitors.
Gibbs' wealth was estimated at $440 million in the National Business Review last year.