Kewell, whose current deal runs out in the summer, has been out of favour since the FA Cup defeat by Barnsley and was not even in the squad that travelled to Italy this week for the Champions League victory over Inter Milan.
Benitez says: "They play two games and he is not playing here, so it is good that he goes with them and plays to get fit.
"I have given him permission because he was not playing for us at the moment. So it is positive for him and his national team that he plays for them.
"The contract will be more difficult for him, if he plays well for Australia and comes back fit he could have a part to play at the end of the season.
"But his situation could become more difficult. I will make a decision at the end of the season."