Gammy's Thai mum lashes out at Australian dad's 'lies'

The Thai mother of an abandoned baby boy with Down syndrome has threatened to sue his Australian biological parents and rejected claims they knew nothing of their son.

The Australian parents are accused of leaving seven-month-old Gammy in Thailand after he and his healthy twin sister were born to local surrogate Pattaramon Chanbua.

Only his sister was brought back to the parents' home in Bunbury, Western Australia.

The couple deny the allegations and claim they had no knowledge of a second child.

Last night 9NEWS revealed that the Western Australian father was jailed in 1998 after being convicted of indecently dealing with a child under the age of 13.

The man's wife said she believes he is a good man regardless of the conviction.

Ms Chanbua hit back at the couple's claims that they are not Gammy's parents on ABC's 7.30 program last night.

"This is not the truth," said Ms Chanbua, 21, who is now Gammy's full-time carer.

Ms Chanbua said the Australian couple asked her to abort the boy after he was diagnosed with Down syndrome in the womb.

She cried, refused and told them she would take care of both the babies if they did not want them.

They eventually agreed to take the girl.

"If they don't know about the twin then they wouldn't be crying the day that they took the girl out from the hospital [and home to Australia]," Ms Chanbua said.

Ms Chanbua said she had forgiven the Australian couple but had now changed her mind and might sue them.

"I am very upset that the father said like that. I really want him to come to Thailand to see me … I would like to talk with him in front of the media," she said.

"No one can lie and the truth will come in the public … people who don’t know me will think that I’m a bad person."

Gammy's story could prompt the Australian government to look closer at surrogacy laws, with the Department of Foreign Affairs already examining practices in Thailand.

Commercial surrogacy, where a woman is paid a fee to carry a child, is illegal in Australia.

However, if there is an agreement for the biological parents to cover just medical and other reasonable costs, the practice is legal.

Gammy's story has prompted donations of more than $200,000 to pay for his future in Thailand.