Oh look....
The information is on the Thai GB website
"Thai Airways accommodates the varying requirements of our passengers, such as communication, visual difficulties or mobility concerns. Please telephone reservations, on 0844 561 0911, giving as much notice as possible.
Wheelchairs are available free of charge and should be requested through our reservations department at least 24 hours prior to departure."
Can't find it on the AU website so just phone them in Australia and ASK for help.