Can a father be made to pay maintenance for his daughter who is 100% disabled, after she is 18 years old?

Yes, in certain circumstances! According to section 4(2) of the Family Law Amendment (Maintenance) Act of 1959, a person can be obliged to pay maintenance for a child who has reached adulthood, providing he,himself, has enough to supply his own needs, those of his spouse/partner and his minor children, and, providing that his disabled adult child "is unable to supply his needs from work,his property or from another source."