The 12 deaths have occurred in little more than three weeks, in what has been a horror start to the Hendra season.
New South Wales biosecurity officials said tests had confirmed a horse who died in northern New South Wales last week had the virus.
The animal had died suddenly on a Lismore property on Thursday - the fourth horse in the state to contract the disease this season.
One other horse on the property has been quarantined but remains well at this stage.
About 80 horses and around 40 people are being monitored for the disease as a result of the cases in recent weeks.
It normally takes six weeks before people and horses are given the all-clear.
Hendra is capable of being passed on to humans once the infection is established in a horse. Seven cases in people have been recorded since the virus was first identified in 1994, and four have proved fatal.