The 32-year-old, who is currently out of contract after being released by Turkish side Galatasary, is struggling for match fitness.
Former West Ham defender Lucas Neill was also released by Galatasary, although he has been deemed fit enough to play against Wales.
"There is a slight difference between the two of them, one is fit and one is not fit," said Australia coach Holger Osieck.
"Lucas has done a lot of hard work. He is in good shape but Harry is not in very good shape."
Australia remain hopeful that former Leeds playmaker Kewell will be fit for their World Cup qualifier against Thailand in September.
The Socceroos play Wales at the Cardiff City Stadium on Wednesday, August 10.