Also returning to the squad is loose forward Santiago Grondona, who was injured in Argentina's final warm-up game ahead of the World Cup in France.
Petti’s return means 20-year-old Efrain Elias, who won his first cap in the surprise 38-20 win over New Zealand in Wellington two weeks ago, drops out of the squad.
Among the backs, Rodrigo Isgro returns after competing for Argentina’s sevens at the Olympic Games in Paris, while Ignacio Mendy and Justo Piccardo are recalled after being in the squad in July for tests against France and Uruguay.
Mendy scored three tries against Uruguay in a 79-5 win in Montevideo.
Lock Luciano Asevedo moves up from Argentina’s under-20 team and is included in the squad as a development player.
Matias Moroni tore a muscle in his left hip region against the All Blacks in Auckland last Saturday and misses out while backup full-back Martin Bogado is dropped.
Argentina’s shock win over the All Blacks in their opening game of the southern hemisphere championship was followed by a sobering 42-10 loss to the same side in Auckland last weekend.
The Pumas host Australia in La Plata on Aug. 31 and in Santa Fe on Sept. 7.
Contepomi said it would be an enormous challenge against the Wallabies, who lost their opening two matches at home against South Africa.
“Although they did not have the expected start, I know and respect (coach) Joe Schmidt a lot and I know all the potential that the Australian team has," he added.
"The tournament is long, only a third of the way through, and we will try to be more consistent and competitive in the upcoming matches."
Argentina squad:
Backs: Tomas Albornoz, Lautaro Bazan Velez, Gonzalo Bertranou, Mateo Carreras, Santiago Carreras, Santiago Chocobares, Lucio Cinti, Santiago Cordero, Bautista Delguy, Gonzalo Garcia, Rodrigo Isgro, Juan Cruz Mallia, Ignacio Mendy, Matias Orlando, Justo Piccardo
Forwards: Luciano Asevedo, Eduardo Bello, Agustin Creevy, Thomas Gallo, Juan Martin Gonzalez, Santiago Grondona, Marcos Kremer, Tomas Lavanini, Pablo Matera, Franco Molina, Julian Montoya, Joaquin Oviedo, Juan Bautista Pedemonte, Guido Petti, Pedro Rubiolo, Ignacio Ruiz, Joel Sclavi, Lucio Sordoni, Mayco Vivas.
