No matter the capacity of your son or daughter with an intellectual or multiple disability, your official parental powers are taken from you by common law! At 18, your family member is an adult by common law!
You no longer have any rights!
Problems can manifest in many ways. Banks may not accept you represent your family member, Health funds may not accept you, service providers can be difficult.
If you wish to continue to protect your vulnerable family member, as you did before they were 18, in the State of Victoria, you need “Plenary Guardianship and Administration” under the Guardianship and Administration Act - through VCAT.
If your family member has sufficient capacity, you can get Enduring Power of Guardianship and Enduring Power of Attorney (Finance and Medical). Check it out with your lawyer!