The Australian biological father of a Thai-born baby with Down syndrome who sparked a global debate over surrogacy was accused on Tuesday of trying to access money donated to help the boy.

David Farnell, a convicted sex offender, and his wife, Wendy Li, made international headlines last year when they were accused of leaving their baby, Gammy, in Thailand with his surrogate mother because of his condition.

They took his healthy twin sister, Pipah, back to their home in Perth.

The scandal led to an outpouring of support for Gammy, with more than $187,000 donated from well-wishers worldwide.

Gammy lives with his Thai surrogate mother, Pattaramon Chanbua, outside Bangkok.

Hands Across the Water, the charity managing donated money, said Farnell was trying to get the cash.

"Hands Across the Water was made aware several weeks ago that the Farnells were taking action in relation to accessing the Baby Gammy Special Appeal Fund," the group's founder and chairman, Peter Baines, said in a statement.

"We have no information as to the justification the Farnells, or their legal team, are making to support the application for access to the funds."

He added that the charity will do all it can to defend the action "to ensure the money donated remains for the care of baby Gammy, which we believe was the intent of those who so generously donated the funds".

In an interview with the Australian Broadcasting Corp, Pattaramon, who was paid to carry the twins, said she was shocked by the attempt to access the money.

"He does not deserve or have any rights to the fund as he abandoned Gammy in the first place," she said.

"People donated money for Gammy and not for anyone else. Even though I am Gammy's mother, I don't have any right to take it. I want to ask him: 'Who do you think you are? What made you think you have the right to take it?' "

The Farnell's were not available for comment.

They previously denied abandoning the boy, claiming they had wanted to bring him home and left Thailand fearing the surrogate mother would seize their daughter.