The ombudsman also questioned how a high-profile prisoner like Williams could be killed inside Victoria's highest-security prison unit and criticised the culture among prison officers at Barwon Prison.
George Williams is now claiming his son repeatedly asked to be moved away from Johnson, a fact he failed to mention in evidence at Johnson's trial.
Victorian Premier Ted Baillieu said the state would fight any compensation bid.
"Everyone is entitled to their rights, but the state would vigorously defend any such action," he said.
The Williams case has parallels with that of triple murderer Gregory Brazel, who was bashed in prison in 1998 and got A$12,000 damages and A$80,000 for legal bills from the state. One of his attackers was Johnson.
- AAP