One of the victims of a convicted child sex offender in Australia who recently fathered twins by a Thai surrogate said Saturday he should not be allowed to have custody of children.

David Farnell's past came to light after the surrogate accused the 56-year-old electrician and his wife of abandoning one of the children - a boy called Gammy who has Down's Syndrome.

"We need to stop putting the perpetrators first and allowing different laws only at different times, because there are so many innocent babies and children that are abused day in day out. Its disgusting," the West Australian newspaper quoted the woman identified only as Sarah.

In 1997, Farnell was convicted of sexually abusing Sarah and her sister when they were aged 7 and 10. His three-year prison term was extended by 18 months when he was found guilty of sexually assaulting an 11-year-old girl.

"It is my belief he should not have any children - not just him, all paedophiles," Sarah said.

The baby Gammy case prompted an public outcry and put a spotlight on Thailand where thousands of foreigners seek the services of surrogate mothers each year.

The Farnells are raising his sister Pipah in Australia.

Australian child protection services launched an investigation after media reports of Farnell's crimes against children.