Viduka has two years left on his Newcastle contract after this season, and with the 2010 World Cup taking place soon after the expiry of his current deal.
He could bid farewell to football at the tournament in South Africa after a goal-laden career.
Viduka: "It is very difficult for me to make up my mind because I still love playing for Australia.
"I loved the last World Cup and, of course, I would love to be part of the next one."