"I know what the game means for me but more importantly it's a chance for us to revenge the defeat in World Cup," said Williams.
"I'm really excited and really looking forward to the game but it's another chance for us to progress as a nation and there is a lot in this game.
"We are determined to progress in the World game and this is a chance to face one of the best and move on.
"We know we have come a long way and we are proud of our achievements but we wanted to do more.
"The focus is now on preparing for the Six Nations, which I believe we can win again and so playing one of the best teams in the World is ideal preparation."
If selected Williams will win his 87th cap at the Millennium Stadium and he believes he is leaving the International game at the right time and is leaving the future of Welsh rugby in very good hands.
"It is the right time," he added.
"There are a lot of youngsters coming through that will be playing for Wales for a very long time.
"There are a lot of guys coming through in my position who are more than capable of playing at International level.
"And its great to see the future of Welsh rugby looking so good.
"The closer it gets to the day, the more sad I will be, but it is the right time - it's my decision and I'm just excited about the day to be honest.
"I'll have a lot of friends and family there and it will be great to run out there again and it will be an even greater day when we beat Australia.
"I've got no second thoughts. I did think the closer it got the harder it would be but it just feels right, you just know when its time."