The biological father of Gammy, the baby at the centre of a surrogacy dispute last year, is trying to access donations raised for the child's medical costs, a charity says.

The case made international headlines last year after convicted sex offender David Farnell and his wife Wendy Li abandoned their son in Thailand because he has Down syndrome and took only his twin sister back to Australia.

Gammy lives with his Thai surrogate mother, Pattaramon Chanbua, who depends on the money donated to charity to cover his medical costs.

The founder of the Hands Across The Water foundation, Peter Baines, told the ABC that Mr Farnell was now trying to access the money raised.

"It's perplexing. I don't understand it on any level," Mr Baines said.

"The funds were donated by everyone because of the alleged actions of Mr Farnell, and to think he believes he has some right of claim over it ... I find it perplexing."

Mr Baines said more than 6,000 people had donated around $235,000 to the foundation since details of the Gammy case were made public.

He said he was not certain how much of the money Mr Farnell was seeking.

"The money was donated through the goodwill of people from not just Australia, but across the world, for the immediate care and for the long-term care of Gammy," Mr Baines said.

"We've taken all steps we can as a charity to ensure that 100 per cent of those funds that we donated go directly to Gammy now and for the future."

Get more stories like this Subscribe to get ABC News delivered to your email, including alerts on major breaking stories, daily wraps of the top issues and analysis and the latest stories on topics that interest you.

Speaking to the ABC, Ms Chanbua asked if Mr Farnell had "gone insane to think like this".

"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?'"

So far the money donated has been used to buy a home for Gammy's impoverished family and monthly payments are made to his mother for his ongoing medical and welfare needs.

Mr Baines said it was estimated the remaining funds would cover Gammy's expenses for another five or six years.

"I certainly don't think [the funds] were donated by anyone thinking that Mr Farnell would at some point have access to them, and we'll do all we can to prevent that," he said.