She had been staying at the farm as a guest and had asked to ride the farm machine, Mr Wilkinson said.
He was not with her when the accident happened and didn't know whether she had worn a helmet.
"I have since spoken to her in hospital and we were very relieved to hear she had no other serious injuries, apart from a nasty gash above her knee. Thankfully, her back and neck are fine and there's really nothing that time won't heal."
Mr Greenberg said a Wellington Free Ambulance paramedic was aboard the rescue helicopter, which arrived at the scene at the same time as a road ambulance.
Ms MacPherson had received serious leg injuries, cuts, and bruises and was flown to Wellington Hospital Emergency Department in a serious condition, he said.
Mr Wilkinson expressed his gratitude to the Wellington Free Ambulance and the Life Flight Trust Westpac Rescue helicopter for their work in treating and rescuing Ms MacPherson.