Thousands of healthy greyhound puppies are disappearing, presumed killed, every year, but their deaths are not reported or investigated by the $144 million greyhound racing industry.

Shocking details about puppy farming and the mass killing of the pups have emerged as a record number of people and organisations told a NSW parliamentary inquiry about the dark practices of the greyhound industry.

In one submission, a former industry participant, who did not want to be identified because he said he feared for his safety, said: "I actually found a brown sack one day, when washing my hands in the river that ran through the property, full of dead newborn pups."

In 2011, up to 3440 puppies were born in registered litters but disappeared before they were named. Naming is a prerequisite for the dogs to race.

Rescue and adoption group Amazing Greys says the industry is one of the largest puppy mills in Australia and relies on breeding a huge excess to find a few champions. ''The industry is characterised by routine killings of puppies and dogs, greed and profits,'' its submission said.