Hi Bec, and welcome to the forums.
A voluntary patient can be admitted to a hospital but it is free for them to leave whenever they want, but if he threatens to physically hurt your nanna then the police can be called and then taken to a hospital for an assessment.
If your nanna says something to her doctor, who is also your grandfather's doctor then he can make a determination and decide whether or not he needs to go to the hospital.
The aim is to support people to recover, including giving them clear rights to make decisions about their own treatment.
Each State has different rules but you can take the initiative to get help in order to prevent a tragic outcome and perhaps you could go with your nanna to her doctor and discuss the situation.