Surgeons in PNG were able to remove the diseased bone, but her leg was left twisted and shorter than the other.

"They got rid of my infected bone [but] the doctor told me that I wouldn't be able to walk normally and my leg wouldn't grow as normal," Sonia told the ABC.

She had to use crutches, found walking to school difficult and often felt lonely and sad.

"I didn't get to play or go and swim because they didn't want the infection to get worse," she said.

"So I stayed home and didn't do much. It was very upsetting."

That was just the way it was until last year when the Children First Foundation brought Sonia to Australia to straighten and lengthen her leg.

She underwent a series of complex orthopaedic treatments at Melbourne's Epworth Hospital.

Children First has been giving children from developing countries the opportunity to receive life-changing and sometimes life-saving surgery since 1999.

The details of the treatment are difficult to read about, let alone undergo.

They involved a team of surgeons cracking two bones in Sonia's leg and using devices that slowly pulled the pieces apart over a period of months as her body tried to fuse them back together.

Professor Minoo Patel, who led the team, said the technique was originally developed by a Soviet doctor, Gavriil Ilizarov, in the 1950s.

"He discovered that if you crack a bone and gradually start stretching it, the bone will grow," he said.

"The body thinks it's a fracture and desperately tries to heal it. You trick the body by stretching it out.

"What is amazing is that when you stretch the nerves and the arteries and muscles, they also start developing and add new cells and regenerate.

"It's just absolutely amazing what the body can do."

Professor Patel's team used a frame around Sonia's lower leg that stabilised the tibia with pins and an external mechanism to stretch the bone.

For the femur, they inserted an expandable rod with a tiny gear system inside the hollow bone. It was activated with a magnet.

Professor Patel said the stretching process was "quite painful".

"There's intensive physio involved," he said. "Those muscles stretch as they tighten. They can only stretch so much. It becomes tighter and more painful.”

In the end, Sonia's leg grew a total of 16 centimetres.

Professor Patel said Sonia was a "determined" patient.

"She was determined as hell she wanted to finish the whole treatment," he said.

"She wanted the full length. She wanted to be able to walk without crutches. She wasn't going to give up.

"She had a once-in-a-lifetime opportunity and she was going to make the most of it."

Professor Patel said such operations were a big team effort, with everyone including himself, the anaesthetist, the nurses and others all working without charge.

Medical equipment worth thousands of dollars was donated by suppliers.

"It's everybody's effort that counts," he said. "It's not just one person. It's teamwork. And at the end of the day, I think everybody feels better for it."

Sonia is now back in Port Moresby where she will quarantine for three weeks before being reunited with her four siblings and parents.

Her dad, Bui, said the 18 months apart had been difficult, but worthwhile.

"The family is very happy," he said. "We missed her, but we are happy that she went for a purpose to get healing."

Sonia plans to finish her high school certificate, then go to university to study medicine with hopes she can eventually become a doctor.

"I want to give back the hope that has been given to me, to be able to give something," she said.

"I just want to help people the way I have been given the help."