Crazy.
1. There's a My Aged Care application creating user records that doesn't sanitise leading spaces, so any surname can be entered as ' Surname' without error.
2. Meanwhile the My Aged Care wizard allowing you to link the service to MyGov—provided you can confirm your identifying data—refuses to match 'Surname' with ' Surname'.
3. If you call the national My Aged Care helpdesk, they transfer you to the development team and manually update the record.