A state parliamentary inquiry is examining the proposal after the government in December signed a memorandum of understanding with racecourse owner the Australian Turf Club for plans to build more than 25,000 homes on the site.
The plan for the 140-year-old track is a once-in-a-generation chance to boost much-needed housing supply in the area, near Parramatta in the city's west, the government says.
The turf club wants to use the proceeds from the sale - potentially worth up to $5 billion - to invest in other Sydney racing, training and stabling venues.
But some thoroughbred industry players say the plan lacks transparency and want assurances club members will have full voting rights on a sale when they consider the plan later in the year.
Proponents say the sale of Rosehill will ensure the future of the racing industry in NSW. (David Moir/AAP PHOTOS)
Appearing at the inquiry on Monday, Ms Waterhouse said she was "incensed at the attempt to sell Rosehill and hijack - meaning steal - the proceeds".
"I am infuriated by the mendacity of the ATC board and associates," she said, while excluding board members Tim Hale and Caroline Searcy from her criticism.
"We would not be here today to discuss selling the cricket ground, the SCG, or Bondi Beach, but yet we're here to discuss the sale of Rosehill," Ms Waterhouse said, sitting alongside fellow Sydney trainer John O'Shea.
She pointed to the sale of Harold Park, the long-time home of harness racing in Sydney, to developers, which led to the sport's relocation from inner-city Glebe to Menangle, on the city's southwest fringe.
Harold Park was sold on the basis of securing harness racing's future but "zero crowds" attended the sport's new basic, she said.
"So tragic," Ms Waterhouse said, adding that the "elephant in the room" was how to fund racing into the future.
"The ATC has an unsustainable financial model," she said.
Ms Waterhouse holds the record at Rosehill's most famous race, the Golden Slipper, having prepared eight previous winners.
NSW Premier Chris Minns said Ms Waterhouse was entitled to her view when asked on Monday about her opposition to the plan.
"I'm not confident of getting Gai over the line," Mr Minns told reporters.
"I'd just make this point that this is a great opportunity for literally billions of dollars to go into racing in NSW and completely remake the industry."
The inquiry will hear from western Sydney business leaders, who have backed the project and the possibility of an on-site metro station as delivering an economic boon for the region.
It will also hear from Thoroughbred NSW, the peak body for the state's thoroughbred breeding industry, and from Coalition for the Protection of Racehorses, an animal-welfare group.
