The 23-year-old has reinforced Unai Emery's ranks for an undisclosed fee and is believed to have signed a four-and-a-half-year contract until the end of the 2027-28 campaign.
Gauci joins first-choice Emiliano Martinez and backup Robin Olsen in the Villa goalkeeping department, where a space opened up thanks to Filip Marschall's loan to MK Dons.
Gauci arrives in the West Midlands following stints with A-League sides Central Coast Mariners, Melbourne City and Adelaide United, keeping 17 clean sheets in 76 games for the latter club.
The 6ft 4in shot-stopper has made two appearances for the Australia national team and was called up to their squad for the Asian Cup, although he is yet to play a single minute at the tournament.
The Socceroos are in quarter-final action against South Korea on Friday, and Gauci will be integrated into Emery's squad once Australia's run in the tournament comes to an end, assuming that he receives a work permit.
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