Arrowfield, Aquis Farm, Coolmore, Darley, Kingstar Farm, Kitchwin Hills, Newgate, Vinery and Yarraman Park Stud have taken the action in the wake of a confirmed case of Hendra virus in a 25-year-old broodmare near Scone last month.
It was the first time the disease had been detected in the Hunter Valley.
Leading broodmare farms are supporting the policy and many have already vaccinated their stock.
They include Amarina Farm, Bhima Thoroughbreds, Cressfield, Edinburgh Park,Kia-Ora, Gooree Park Stud, Glastonbury Farms, Lime Country Thoroughbreds, Luskin Park, Milburn Creek, Riversdale, Toolooganvale, Tyreel, Valiant Stud and Willow Park Stud.
Scone Equine Hospital, Dartbrook Equine Veterinary Clinic and the Hunter Equine Centre have backed the studs to vaccinate their entire horse population and demand walk-on mares are also treated.
The veterinary clinics say they have a legal responsibility with a zero-tolerance level for exposure of staff, horse staff or the public to any horse with an identified risk of HeV infection.
The confirmed case in the Hunter Valley means there is now an elevated level of vigilance for the potentially fatal disease. Unvaccinated horses will not be admitted to veterinary clinics.