A campaign for a baby with Down's syndrome abandoned by an Australian couple after they discovered his condition has raised more than £90,000.

The boy, named Gammy, was left with his surrogate mother in Thailand - while his parents took his healthy twin sister back to Australia.

Pattaramon Chanbua, from Chonburi province, south-east of Bangkok, had agreed via an agent to be a surrogate for the couple's twins, who were born in December, for a fee of £8,850. After the Australian couple learned of Gammy's condition, however, the 21-year-old woman was left to care for the boy, who also has life-threatening heart problems requiring expensive medical treatment that she cannot afford.

The campaign, called Hope for Gammy, has raised more than £90,000 (A$160,000) to help Pattaramon care for Gammy, who is now six months old.

Pattaramon, who was already a mother to two children, said: "Why does he have to be abandoned while the other baby has it easy? I feel sorry for him. I don't know what to do. I chose to have him, not to hurt him. I love him. He was in my tummy for nine months, it's like my child. I treat him like my other children, never think you are not my child and I don't care for you, never."

She added: "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. 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."

The Australian prime minister, Tony Abbott, described the case as an "incredibly sad story". He said: "I guess it illustrates some of the pitfalls involved in this particular [surrogacy] business."

Abbott said he was yet to be briefed on the matter, but added he would look at "what might be possible" to help the boy.

The Hope for Gammy campaign page carries dozens of comments expressing outrage and disbelief at Gammy's parents. "The healthy baby should be taken away from those monsters," said one. "They obviously have no human compassion."

Another said: "May this selfish and heartless couple be exposed and shamed for this horrible neglect!"

Commercial surrogacy, in which a woman is paid to carry a child, is illegal in Australia, but couples are allowed to use an altruistic surrogate – who receives no payment apart from medical and other reasonable expenses.

A Thai public health ministry official, Tares Krassanairawiwong, confirmed that paying for surrogacy was also illegal in Thailand. "Surrogacy can be done in Thailand, but it has to comply with the laws," she said. "A surrogate has to be related to the intended parents and no money can be involved."

According to the support group Surrogacy Australia, outrage over Gammy's story could lead to a complete ban on international surrogacy in Australia, where more couples choose to go overseas than find an altruistic surrogate at home.

"It's just much easier overseas," its chief executive Rachel Kunde told Agence France-Presse. "There's so much red tape involved [in Australia]."

Kunde added that the details of the latest case were not clear and claimed it was not known whether the Australian couple involved had been told that the boy was alive.

"Our greatest fear is that Australia is going to ban international surrogacy altogether," she said. "We are hoping that the government will make accessing surrogates in Australia easier."