The baby born with Down syndrome to a surrogate mother in Thailand has been granted Australian citizenship.

The twin sister of baby Gammy was adopted by Australian parents David and Wendy Farnell last year, but they left him behind.

His surrogatemother said she had applied for Australian citizenship to safeguard Gammy's future.

David and Wendy Farnell say they did not abandon Gammy | Image: Nine Network

It means the boy, who is now one year old, can get an Australian passport and have access to healthcare there.

He was eligible because David Farnell is his biological father.

Speaking to '60 Minutes' on Australia's Nine Network last August, Mr Farnell denied they abandoned their newborn son.