The first foal out of legendary race mare Winx has sold for a world record $10 million at auction.

The 18-month-old filly, sired by Coolmore Stud's champion stallion Pierro, saw bidding begin at $2 million at the annual Inglis Easter Yearling Sale in Sydney.

She drew international interest but was sold to Winx's part-owner Debbie Kepitis and her Woppitt Bloodstock.

The horse is the most expensive thoroughbred filly ever traded in the world. 

The sale also sets an all-time southern hemisphere record for a thoroughbred yearling, doubling the $5 million paid for Black Caviar's half-brother in 2013 and dwarfing the previous Australian filly yearling record of $2.6 million.

Ms Kepitis said she originally had not planned to buy the filly. 

"We put her up for auction and then in the last few weeks, all of the family we started to miss our daughter, granddaughter," she said. 

"We just decided that as best we could, if we could get her we would."

She said the filly would be trained by Chris Waller, who trained Winx. 

"I am privileged to have been able to secure this filly on behalf of my family to be able to hopefully see if she can get to the racetrack," Ms Kepitis said. 

"If she can't get to the racetrack, she will be an amazing mum.

Winx stole the hearts of the nation as she claimed an Australasian record winning streak of 33 races, including 25 Group 1 wins and a record four Cox Plates.

She amassed more than $26 million in prize money before her retirement in 2019.

The mare delivered a stillborn foal in 2020 and was given a year off breeding before getting in foal to Pierro to produce this filly, born in 2022.

The ABC revealed on Friday that Winx is expecting her next foal by Arrowfield Stud's leading stallion Snitzel.

Tom Magnier from Coolmore Stud, where Winx resides, said the champion race mare was lucky to survive her first birthing scare. 

He said it was a "perfect story" for Ms Kepitis to have bought the filly, after being so understanding and patient during Winx's broodmare career. 

"I knew she was going to make a very high figure because somebody's buying history, it's a classic and it's a collectors item," Mr Magnier said. 

"There's only one person who deserved today and that was Debbie.

"To save the mare, to get the foal, and she's back in foal, it's a dream story."