She captioned the photo "PW²" in a nod to their matching initials.
Celebrities flocked to the comment section to send their well-wishes, including model Megan Gale and former WAG Nadia Bartel.
“Congratulations!!! How wonderful 🥂🍾😘,” wrote Lindy Klim.
“How sweet it is ❤️” added Erika Heynatz.
Many, however, couldn’t get past the size of her diamond sparkler, writing: "💎✨👌🏼," and "The ring!!!! Congratulations!!! 😍😘"
“From Pia Miller's celebratory Instagram post, her ring looks like a 10 carat to 12-carat solitaire oval diamond, set in a platinum band, which would be priced at between US$800,000 (AU$1.10million) to US$1.4million (AU$1.9million),” Mr Kormind said.
Patrick, 55, is the CEO of Hollywood talent agency WME (who represents the likes of Matt Damon) and is estimated to be worth US$440million (AU$630million).
“The oval is a historic diamond cut, originally invented hundreds of years ago, but its more modern iteration was created in the 1950s and is a very classic cut which is currently enjoying enormous popularity,” Tobias continued.
“One of the best-known engagement rings with an oval centre stone is Princess Diana's engagement ring with its central oval sapphire surrounded by diamonds, worn today by the Duchess of Cambridge, Kate Middleton.”
The couple have been separated this year due to coronavirus travel restrictions, with Pia based in Sydney and Patrick currently living in Los Angeles. They reunited briefly in Sydney in September, and again this week in California.
It’s believed Pia has been seeing Patrick since May 2019. Just one-month in, she made their relationship Instagram official, declaring she was “in love” during a trip to Paris.
Patrick was previously married to Lauren Sanchez until their split at the beginning of 2019 when her affair with Amazon's billionaire boss Jeff Bezos was unveiled. They were married for 13 years.
They share joint physical custody of their two children, a son and a daughter.
Pia was previously married to AFL player Brad Miller and was engaged to film producer Tyson Mullane up until April 2019.