It was originally thought Williams would call it a day at the conclusion of the third-place play-off in the Rugby World Cup in New Zealand. But despite scoring a try, Wales lost the match 18-21. Williams has now decided to give Robbie Deans' Australia side one more crack.
"I've decided I am going to make myself available for this game and if selected, I want to play," said Williams, who has 59 international tries to his name.
"If I'm picked I will treat it in the same way I have always treated Wales games, but it will be a sad day for me. In fact, I'll be devastated. However, to bow out in front of Welsh fans and my family and friends would be like a dream."
Williams, who will continue to play for the Ospreys after calling time on his international career, added: "I always said I would know in myself when the time came to stop playing for Wales and that time is very nearly upon me.
"After this last game in December, that's definitely it for me, there'll be no changing my mind."