The RSCPA is concerned that rules introduced by racing authorities to address the issue of wastage in horseracing are not dealing with the problem effectively.
"Wastage" is a term used to describe horses that are bred for racing that do not make it into the sport, or are lost from the industry when they are no longer useful.
Many end up being killed for meat or are left languishing in paddocks.
RSPCA Australia's chief scientist Bidda Jones told Four Corners the fate of many racehorses was still unknown.
"We've got a big problem with the number of foals that are produced by the thoroughbred racing industry every year in Australia," she said.
"That number's actually gone down in the past 10 years, but it's around 13,000 foals that are born every year, and about 11,000 of those end up being registered, so 11,000 go into racing, 2,000 don't. That's immediately an issue in terms of oversupply.
"Out of those 11,000, about 8,500 end up leaving racing, and so where do those end up?"
Over the past few years the racing industry has been introducing measures to try to address wastage.
In 2016, Racing Australia brought in a policy of "traceability", forcing breeders and owners to register horses from birth. Racing Australia must also now be notified when owners initially sell or give away their horses.
Racing NSW's CEO Peter V'landys says his state is taking this even further.
"We're introducing new measures here at New South Wales to go and inspect six months after the horse has been provided," he said.
"We want to make sure it's a fit and proper person who's got the horse, but we're going to go and check and make sure that horse is well fed."
But thoroughbreds can be sold on several times, and are difficult animals to retrain and rehome.
Dr Jones told Four Corners that what ultimately happens to many of them is still a mystery.
"A proportion of those horses will end up going into pleasure riding, but some of them won't, and a large proportion will end up with nowhere to go, and the industry has to find a solution for that that's acceptable to the public," she said.
Footage shows starving thoroughbreds
Four Corners has obtained footage of branded thoroughbreds that have been neglected and starved after retiring from racing.
One mare was originally bred by millionaire businessman Gerry Harvey and raced twice by her owners. She was sold on and ended up emaciated with open sores in a paddock in western Sydney. The mare was rescued by animal welfare officials in 2016, close to death.
Another mare found on the outskirts of Sydney was not so lucky. She was skeletal and starving when she was seized by animal welfare officials but could not be saved. An autopsy found the horse was six months pregnant.
The RSPCA was able to provide vision and photos to Four Corners as legal proceedings against the horses' owners have been finalised. The owners of both horses were convicted and fined.
Four Corners understands there have been dozens of seizures and prosecutions involving ex-racehorses since Racing Australia's rule change in 2016.
Mr V'landys said Racing NSW was doing what it could to monitor ex-racehorses for as long as possible.
"That's why we want to check the horses six months later. We're going to take random checks where we're just going to send a vet or one of our stewards out to see where that horse is," he said.
"It might be 12 months later, it might be six months later, but we want to know how you've looked after that horse.
"We're not going to stop once the horse has been given to somebody else. We're going to expand it to the next level where we want to know if the horse is having a good retirement."
New rule could make things worse
But there are serious concerns that a rule introduced recently by Racing NSW is making things worse for some racehorses.
Last year the state introduced a ban on sending racehorses to knackeries and abattoirs.
Four Corners has produced evidence that this rule is not being enforced.
On one day at the Echuca horse sales on the Victorian border, Four Corners filmed at least two NSW racehorses being bought by so-called "kill buyers".
Mr V'landys said the rule was not a guarantee.
"I can't guarantee it at this stage because unfortunately, knackeries aren't regulated, so you don't know if someone can sneak a horse into a knackery," he said.
"The objective is that no horse that races or is domiciled in New South Wales — [it] doesn't have to get to a racecourse — but if it's born, bred, domiciled predominantly in New South Wales, we'll need to find it a home."
Dr Jones told Four Corners the RSPCA was concerned this new rule could lead to more horses languishing or starving in paddocks.
"If you produce large numbers of animals for a particular purpose, [when] they reach the end of that life and you don't have a home for them, well, you can't then subject them to years, months of neglect," she said.
"Actually, sometimes the better thing to do, the kinder thing to do is to euthanase that animal."
She acknowledged this may seem a surprising view for the RSPCA.
"If you have the choice between a horse being neglected for years because they aren't being cared for properly, or going straight to an abattoir and being humanely slaughtered, you shouldn't be ruling out that option," she said.
"Obviously, the better option is to deal with the problem at the source and not breed so many thoroughbred foals, but I think until we actually have a handle on where these horses are going, making a rule like that is potentially creating a welfare problem, not solving one."