Some of NSW's sickest babies are being flown to Victoria because the state's paediatric heart surgery program is "inferior" and unable to attract the best doctors, a leaked document suggests.

Top paediatric cardiologists believe Sydney Children's Hospitals Network's "two-site model" has hindered progress and program development, forcing them to refer babies needing a heart transplant to the Royal Children’s Hospital in Melbourne.

NSW babies needing heart transplants are being flown to Victoria.

"People don't want to hear about it, but the reality is it's happening, it's a source of shame," a doctor familiar with the matter said.

The leak comes amid escalating tensions between Sydney Children's Hospital at Randwick and The Children’s Hospital at Westmead, with Randwick doctors accusing their Westmead counterparts of sucking up resources in order to become the exclusive provider of heart surgery.