When Ms Pattaramon Chanbua, a 21-year-old food vendor in Thailand's seaside town of Sri Racha, agreed via an agent to be a surrogate mother for an Australian couple for a fee of A$16,000 (S$18,577), she thought it would be a great source of income for her family.

"The money that was offered was a lot for me. In my mind, with that money, one, we can educate my children, two, we can repay our debt," Ms Pattaramon, who has a six-year-old son and a three-year-old daughter, told Australia's ABC in an interview from her home in Chonburi province, south-east of Bangkok.

In December, Ms Pattaramon gave birth to a pair of twins - a boy and a girl.

But things did not turn out as smoothly as everyone had expected.

When the Australian couple discovered the boy, named Gammy by his surrogate family, had Down syndrome, they reportedly abandoned him in Thailand and returned to Australia with only the healthy girl.

Now, Ms Pattaramon is left to care for the boy, who was first reported to have a life-threatening heart condition requiring expensive treatment, but subsequent checks by a Thai hospital had found he had no such condition.

"I don't know what to do. I chose to have him... I love him, he was in my tummy for nine months," she said.

"I want to warn those who are considering becoming a surrogate mother, don't think only about the money," she said.

"If the child is born with an unusual condition or if anything goes wrong, it will become a burden for you and society."

Ms Pattaramon said she approached the surrogacy agency on Facebook early last year because she wanted money to pay off her debts, Australian media reported.

"They (the surrogacy agency) told me to carry a baby for a family that does not have children... They said it would be a baby in a tube," Ms Pattaramon said.

A spokesman for Australia's foreign affairs department told AFP that Canberra was "concerned" by the reports and was in consultation with Thai authorities over surrogacy issues.

Many foreign couples travel to Thailand, a popular medical tourism hub, to use its in-vitro fertilisation (IVF) services despite the unclear legal situation surrounding surrogacy.

Tares Krassanairawiwong, a Thai public health ministry official, said it was illegal to pay for surrogacy in Thailand.

"Surrogacy can be done in Thailand but it has to comply with the laws... A surrogate has to be related to the intended parents and no money can be involved."

Australia's Immigration Minister Scott Morrison said on Sunday a question of "moral responsibility" should determine Gammy's fate.

"I think this is an absolutely heartbreaking story," Mr Morrison said.

The story of Gammy has triggered hundreds to donate to a fundraising website created for him last week.

So far, the "Hope for Gammy" page has raised more than A$200,000.

It also carried scores of comments, many of which expressed outrage at the boy's abandonment by his biological parents.

"May this selfish and heartless couple be exposed and shamed for this horrible neglect!" said one.

"From my children to yours. They are the future and come all differently but always a gift from above," wrote one donor.

Gammy, who has blond hair and dark brown eyes, was admitted to a hospital east of Bangkok because he had difficulty breathing due to pneumonia and his condition had improved, said a spokesman at the hospital. .

Ms Pattaramon's mother, 53-year-old Pichaya Nathonchai, told AFP that Gammy "is a quiet, calm boy ... his mother and I are taking turns to see him at the hospital."

Ms Pattaramon said she was told of Gammy's condition only late in her pregnancy.

She said her doctors, the surrogacy agency and the baby's parents knew he was disabled at four months but did not inform her until the seventh month when the agency asked her - at the parents' request - to abort the disabled foetus.

She told Reuters Television she refused the abortion on religious grounds.

"I'll take care of Gammy on my own. I'll not give my baby to anybody," she told Fairfax Media.

"I forgive them for everything. That is the best thing I can do, forgive... it is best for everybody," she said of the Australian couple.

"I can't blame them... I don't feel upset or angry about them anymore. They might have their own problems too."

Ms Pattharamon said she met the Australian couple only once when the babies were born and knew only that they lived in Western Australia.

She told Fairfax Media that the couple said they were too old to care for twins.

She said the father of the baby, who was in his 50s, "came to the hospital to take care of the girl but never looked Gammy in the face or carried him", Sydney Morning Herald reported.

"He did not buy milk for Gammy. He only bought milk for the girl," Ms Pattaramon said.

"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."

She said the parents agreed to pay her another 150,000 baht (S$5,820) to keep Gammy.

But in a twist to the story, the couple had denied they were the parents of baby Gammy.

When contacted by 9News, the couple, who have not been identified, admitted they had a baby girl about the same age as Gammy and that they had used a surrogate. But they said she did not have a twin brother.

The man told the ABC that the clinic had told them there was only a baby girl - not a boy.

The couple described their surrogacy experience as "traumatising".

Days later, reports emerged in Australia that the man was convicted for sexually molesting three girls. ABC reported that he was jailed for three years for molesting two girls under the age of 10 when he was in his 20s. He was also accused in 1997 of another six charges of indecently dealing with a child and reportedly imprisoned for 18 months.

Australian authorities are investigating the man to determine whether his young daughter is at risk.

Meantime, Ms Pattaramon, who said she has not been fully paid since the children were born last December, said she would raise Gammy on her own.

"I am glad that in this unlucky situation there is a blessing that we are together," she said. "I never thought to abandon him. I love him as my own baby, he is my baby. I love him very much," she said.

She said she wants to meet the surrogacy agent and the Australian couple.

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