SYDNEY - An Australian couple accused of leaving their Down Syndrome baby in Thailand while returning home with his healthy twin sister deny that they knew about the baby boy, Australian media reported.

In a dramatic new twist to the story that has captivated the world's attention, the ABC is quoting the unidentified father as saying a Thai surrogacy clinic doctor only told them about the baby girl.

The man was quoted as saying he had a lot of trouble with the surrogacy agency and had been told it no longer existed, Sydney Morning Herald reported.

But the baby boy Gammy's Thai surrogate mother Pattharamon Janbua, 21, said earlier the parents told her they could not take Gammy because they are too old to care for twins.

She claimed the father, who is in his 50s, "came to the hospital to take care of the girl but never looked Gammy in the face or carried him".

"He did not buy milk for Gammy. He only bought milk for the girl," Pattharamon said from a Thai hospital where Gammy is critically unwell. The baby is suffering from a life-threatening heart condition.

"The twins stayed next to each other but the father never looked at Gammy... not one bottle of milk did he give Gammy," she said.

"I could say he never touched Gammy at all."

Pattharamon, who is known as "Goy," said she wants to meet with the surrogacy agent and the Australian couple.

"I would like to ask them who is telling the lie," she said.

Pattaramon gave birth to Gammy and his twin sister last December after she was reportedly paid A$16,000 (S$18,600) to be a surrogate mother.