Seven-month old Gammy has been at the heart of a surrogacy scandal since it was revealed his biological father David Farnell and his wife left the baby with surrogate mother Pattaramon Chanbua, taking only his healthy twin sister Pipah back home to Australia.
In their first interview since sparking a global controversy, Farnell, 56 -- a convicted child sex offender -- and his wife Wendy claimed they had wanted to bring their son home but left Thailand in fear Pattaramon would seize Pipah.
"We have been trying (in Australia) to make sure first that Pipah is safe and no one can take her away from us," he told Channel Nine on Sunday, explaining that as she was born in Thailand she was not yet legally Australian.
"When we know 100 percent that she is safe with us, we can go and try to get our boy back."
But Mrs Pattaramon, 21, said they could never take her child, who she says was conceived after another Thai donor's egg fertilised by Farnell was implanted in her on the promise of a US$14,900 fee.
"If he wants to get my baby, it will be impossible for sure. No matter how hard he fights, there is no way that I will let that happen," she told AFP by telephone Tuesday.
"If he wants to see the baby, I will let him see, but he has no right to take my baby."
There have been various contradictory versions of the events that led to Gammy being left behind in Thailand.
The Farnells, from Bunbury in Western Australia, previously said they were told Gammy had a congenital heart condition but not Down's syndrome, and left him because doctors said he would not survive.
Mrs Pattaramon maintains the Thai agency who managed the surrogacy said the couple wanted her to have an abortion -- illegal in Thailand -- once medical tests revealed the boy had Down's syndrome, but she refused.
During the interview Sunday, Farnell denied he and his wife had asked her to have an abortion but said it crossed their minds, also admitting they had not tried to contact Gammy since they left Thailand to check on how he was.
Doctors have since cleared the boy of a heart condition and he lives with his surrogate family in Chon Buri.
Following reports on the scandal, the Thai government has proposed tighter controls on commercial surrogacy .
Canberra has since urged it to allow for a transition period before implementing any ban on commercial surrogacy to protect earlier arrangements made by Australians.