You can stay until your health deteriorates and you become a burden to society is the position of the Australian Government.
Bridging Visa's for this category are seen as indefinite as there is a 30 year wait for the Visa approval and not only do you need a Medical when you apply for the Visa (which being an aged parent visa you would be already getting on in years when you applied) so imagine at the age of 60 you get a medical and apply and you are all good to go...... tick tock tick tock 15 years later when you are 75 you develop a medical problem and now with the Government linking your medical data (data retention and analysis that things like Robo Debt run on) and apply it to anyone with a bridging visa.
On the otherhand, you have been able to bring a parent over on a tourist visa, apply for the aged visa on-shore and be happy to sit and wait on the briding visa indefinitely.
What would an Imigration Agent do?
Firstly the case will be appealed to the Department and then the Minister. This process will take some time and you would be appealing on compassionate grounds.
With the current mood of the government, one would speculate that there was no compassion to be found or they would have found it on their own without need to be reminded that you were about to deport a 92 year old grandmother to go home and die alone.
Next the matter would be appealed to the Administrative Appeals Tribunal and you would fight it all the way to the High Court if necessary and the time that it would take to do that would give you a valuable few more years together a family.