The 34-year-old will line up in New Zealand for her fourth World Cup, the only new face in the squad from last month's series against the Black Ferns.
Williams was part of the Australian team which won the World Rugby Sevens Series title in May and then backed up to lead her country to a gold medal at the Commonwealth Games in Birmingham.
She is currently in Cape Town for the World Cup Sevens and following that will join the 15-a-side team for that World Cup.
Teenage winger Bienne Terita missed the Commonwealth Games but was recalled for the sevens tournament before also linking with Wallaroos.
Terita made her 15-a-side Test debut last month, scoring two tries in their 22-14 loss to New Zealand at Adelaide Oval.
Wallaroos coach Jay Tregonning confirmed the squad as well as their captain, Shannon Parry, a fellow sevens convert.
"The squad has experience and depth, and we are looking forward to showing our continued improvement at the Rugby World Cup," Tregonning said in a statement.
"Although disappointed, the players that have missed out have continued to train hard in their state-based training sessions and are committed to helping make their teammates stronger.
"We wish the sevens team all the best in their World Cup in Cape Town this weekend and we are excited to have Sharni and Bienne as part of our squad at the conclusion of the tournament."
The Australians open their campaign against the hosts on October 8 looking to improve on a sixth-placed finish at the last tournament in 2017.
As well as New Zealand they face pool matches against Scotland (October 15) and Wales (October 22).
Wallaroos: Forwards - Liz Patu, Emily Robinson, Bree-Anna Cheatham, Madison Schuck, Bridie O'Gorman, Eva Karpani, Ashley Marsters, Adiana Talakai, Tania Naden, Michaela Leonard, Sera Naiqama, Atasi Lafai, Kaitlan Leaney, Shannon Parry (capt), Emily Chancellor, Grace Hamilton, Grace Kemp, Piper Duck. Backs - Iliseva Batibasaga, Layne Morgan, Arabella McKenzie, Trilleen Pomare, Siokapesi Palu, Cecilia Smith, Georgina Friedrichs, Sharni Williams, Mahalia Murphy, Ivania Wong, Bienne Terita, Maya Stewart, Pauline Piliae-Rasabale, Lori Cramer.
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