An Australian couple at the centre of a Thai surrogate scandal Sunday denied they deliberately abandoned their baby son because he had Down's syndrome and said they would fight to get him back.

David Farnell, 56, a convicted child sex offender and the biological father of the boy, Gammy, told Channel Nine he and his wife Wendy had "wanted to bring him with us".

It was their first interview since sparking global controversy after leaving the boy in Thailand with his 21-year-old Thai surrogate mother Pattaramon Chanbua and taking only his healthy twin sister Pipah back home.

"We never said you (Pattaramon) can have this baby, no matter what," said an emotional David Farnell.

The couple claimed Pattaramon wanted to keep the girl and said they left Thailand without Gammy because "we were getting scared that we would lose her too and we had to try and get her out".

The Farnells, from Bunbury south of Perth, Western Australia, previously claimed they were told Gammy had a congenital heart condition but not Down's syndrome, and left him because doctors said he would not survive.

Gammy, now seven months old, has since been cleared of a heart condition by Bangkok specialists, an Australian charity which has raised more than US$240,000 (RM769,800) for the infant said on Friday.

The charity, Hands Across The Water, said the boy had been released from hospital -- where he was being treated for a lung infection -- and was living in Bangkok with his family.

'We never abandoned him'

David Farnell claimed Pattharamon said: "If we try to take our little boy, she's going to get the police and she's going to come and take our little girl ... and she's going to keep both babies."

Pattaramon has said she agreed to carry another Thai donor's egg fertilised by the Australian man in exchange for around $14,900.

An agency, which she refuses to name for legal reasons, acted as the go-between.

She claims the agency told her the Farnells wanted her to have an abortion -- which is illegal in Thailand -- once medical tests revealed the boy had Down's syndrome, but she refused.

"We never abandoned him, we never said to the surrogate mother 'have an abortion'," said David Farnell, although he admitted it crossed their minds.

"Because he has a handicap and this is a sad thing. And it would be difficult, not impossible, but difficult."

Farnell also admitted the couple had not tried to contact Gammy since they left Thailand to check on how he was.

"We have been trying (in Australia) to make sure first that Pipah is safe and no one can take her away from us," he said, 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."

Their comments came as Canberra urged Thailand to allow for a transition period before implementing any ban on commercial surrogacy to protect earlier arrangements made by Australians.

The Thai government is proposing tighter controls on commercial surrogacy in the wake of the Gammy scandal.