The RSPCA has uncovered a mass grave containing the remains of nine greyhounds on the western Sydney property of a licensed trainer.

WARNING: This story contains a graphic image of dog carcasses in a grave.

Twelve other greyhounds were removed from the property suffering emaciation, with parasites, pressure sores and severe dental disease.

RSPCA NSW chief executive Steve Coleman said a detailed autopsy would be performed on the dead animals to help decide about prosecution.

"What we came across was quite grisly," he told ABC Radio Sydney.

"You can just imagine trying to literally dig out with a shovel a number of greyhounds that had died, allegedly in the last 12 months or so."

The remains were discovered after a tipoff to inspectors on July 3.

Officers initially seized nine dogs and left detailed veterinary instructions in relation to the remaining three, however those animals were also surrendered when the instructions were ignored.

Racing Minister Paul Toole said in a statement the allegations were "extremely serious".

"There is zero tolerance for such abhorrent behaviour and those who engage in it can expect to be removed from the industry and face criminal prosecution."

The discovery comes two years after the then Baird government introduced a ban on the greyhound racing industry, before backflipping a few months later.

A spokesman for the Greyhound Welfare and Integrity Commission said it was also providing support to the probe.

Animal rights organisation PETA condemned the discovery and said the industry has had plenty of opportunities to implement reform.

"This most recent investigation confirms that the battle against the abuse of animals for entertainment and financial gain continues," spokeswoman Emma Hurst said.

Nine dead dogs uncovered by RSPCA inspectors at the western Sydney property of a licensed trainer. ( Supplied: RSPCA )

Sick dogs recovering, could be rehomed

Mr Coleman said the 12 sick dogs were responding well to treatment, but described their recovery as "a slow process".

"There's almost a sixth sense with them, that they understand they're being treated properly and with care — and they respond accordingly," he said.

However, he warned it can be difficult to rehome a greyhound depending on each animal's racing background.

"To make the transition into someone's home is not easy, it's certainly not impossible."

The case is the fifth instance of alleged animal cruelty involving greyhounds since the racing ban was announced.

NSW became the first state to ban greyhound racing in 2016 after a Four Corners investigation that uncovered shocking instances of live baiting and animal cruelty.

The RSPCA have rescued 12 greyhounds from the property who are starving and sick. ( Supplied: RSPCA )

A government inquiry found overwhelming evidence of systemic animal cruelty, including mass killings.

The industry fought the ban, arguing innocent trainers would lose their livelihoods.

In October 2016, then-premier Mike Baird reversed the ban, claiming the majority of the community wanted the industry to have a second chance.

The controversial move was considered a watershed moment and was later followed by other smaller policy backdowns.

The NSW Opposition, who put up a huge fight against Mr Baird's ban, has now called on the Government to act.

"We have always said there should be no quarter given to those who break the rules," Acting Opposition Leader Michael Daley said.

"Those people in the industry who break the rules should be thrown out of the industry and face prosecution under the law.

"The Government should investigate these current allegations as a matter of urgency."

Mr Baird announced his shock resignation from politics in early 2017, saying he wanted to spend time with his family.

He had suffered a sharp drop in his approval rating following the greyhound racing ban and backflip, lockout laws and council amalgamations.