Fa'amausili joins the Waratahs from the Melbourne Rebels and Pieretto from the Glasgow Warriors on a week-to-week basis.
Both players may return to their respective clubs this season if needed.
The duo's arrival is a welcome boost for the Waratahs, who have been hit hard by injuries, particularly in the front row, with five of the club's full-time contracted props sidelined, including Wallabies star Angus Bell.
Fa'amausili is a 27-year-old tighthead prop who was born and raised in Melbourne.
He made his provincial debut for the Rebels against the Sunwolves in 2018 at AAMI Park, before being selected for Australia for the Argentine leg of the Rugby Championship in 2022.
He was awarded his long-awaited debut for Australia in the second Pumas Test at San Juan, and has gone on to earn six further caps.
Despite arriving from Glasgow, Pieretto is no stranger to the Super Rugby competition, after enjoying a successful spell with the Jaguares in his native Argentina.
Argentine international Enrique Pieretto is no stranger to the Super Rugby competition. (Mick Tsikas/AAP PHOTOS)
The tighthead featured 17 times for the franchise on their run to the 2019 Super Rugby final, with New Zealand heavyweights the Crusaders denying them a historic title.
A member of Argentina's 2019 Rugby World Cup squad, Pieretto played in the Pumas' pool-stage victory over the USA in Kumagaya as part of a renowned scrummaging unit.
The 29-year-old was also selected for the world's most famous invitational side in November 2019, representing the Barbarians against Fiji at Twickenham.
"Both Pone and Enrique give us experience and size in the crucial prop position where we have been hit by significant injury numbers," said Waratahs coach Darren Coleman.
"Both have played at the highest levels of the game and are hungry for opportunity."
Fa'amausili's addition is a historic one for the Super Rugby Pacific competition, as it is the first time a player-loan like this has occurred between clubs.
"A genuine thank you to both the Melbourne Rebels and the Glasgow Warriors who have worked in a very co-operative and sympathetic manner to allow this," Coleman said.
Fa'amausili has already arrived at Waratahs HQ for training this week and is in line for selection against the ACT Brumbies at Allianz Stadium on Saturday.
Pieretto will arrive from Scotland on Thursday after his visa application was approved on Monday.Â
