British and Irish Lion Roberts was ruled out for six months after having an operation on a troublesome knee injury in April, but Wales will not miss the physicality he brings to the centres as his 12-cap replacement is a similarly robust 101 kgs.
Williams lines up in the huge Welsh backline which left European defences in tatters during their Grand Slam success alongside outside centre Jonathan Davies and wingers George North and Alex Cuthbert, who all weigh in at over 100kgs.
Openside flanker Sam Warburton will lead Wales for Saturday's Test at Lang Park after being forced from the field against France in the Six Nations decider in March with a nerve problem in his shoulder.
With the Wallabies suffering numerous injury problems, the tourists are expected to challenge closely for the series but they have a poor record in the southern hemisphere and have not beaten their opponents in Australia since 1969.
To break that losing run, caretaker coach Rob Howley, in charge after Warren Gatland injured his legs in an accident at home, has made three changes to his pack for their first Test since they beat France in March.
Locks Luke Charteris and Bradley Davies pack down in the second row, with British and Irish Lion Alun Wyn-Jones dropping to the bench and Ian Evans unavailable because he is getting married this weekend.
Scarlets hooker Ken Owens takes over from Matthew Rees in the front row in a strong pack that is likely to attack Australia in their traditional Achilles heel, the scrum, which was wobbled by Scotland in a 9-6 defeat on Tuesday.
Wales lost to Australia 21-18 in the third-place play-off at the World Cup in October and the Wallabies proved to be party-poopers in winger Shane Williams's Testimonial in Cardiff in December by recording a 24-18 win.
Fly-half James Hook, who missed a simple kick in the narrow World Cup defeat, is named on the bench for Saturday's Test with Rhys Priestland charged with calling the shots in the pivotal position.
Hook, who started at inside centre in the 30-21 win over the Barbarians in Cardiff on Saturday, is joined on the replacements bench by uncapped centre Ashley Beck.
"We have created some winning momentum, we have a recent home win against the Barbarians to take into this game and have built a certain amount of confidence within the squad," Howley said.
"We are well prepared and looking forward to the challenge this weekend."
Team: 15-Leigh Halfpenny, 14-Alex Cuthbert, 13-Jonathan Davies, 12-Scott Williams, 11-George North, 10-Rhys Priestland, 9-Mike Phillips; 8-Toby Faletau, 7-Sam Warburton (captain), 6-Dan Lydiate, 5-Luke Charteris, 4-Bradley Davies, 3-Adam Jones, 2-Ken Owens, 1-Gethin Jenkins.
Replacements: 16-Matthew Rees, 17-Paul James, 18-Alun Wyn Jones, 19-Ryan Jones, 20-Lloyd Williams, 21-James Hook, 22-Ashley Beck.