"The success of surgery depends on where the twins are joined and how many and which organs are shared, as well as the experience and skill of the surgical team".
The twins, who were delivered by C-section, flew to Australia last month for the surgery that would finally divide them.
Previously, medical experts were not sure if the girls also shared a bowel, which could have complicated surgery.
Surgeons in Australia have separated conjoined twins from Bhutan in a life-changing operation.
"It is a relief and it's also a joy", Dr Crameri said following the operation.
As well as the issues with mobility and comfort, Lodge said the twins had recently been losing weight, which had been a concern to doctors who are now observing them closely.
They headed into the theatre at 8am on Friday, with doctors administering the anaesthestic about 8.45am.
In 2009, doctors at RCHM successfully separated conjoined Bangladeshi twins, Trishna and Krishna in a 25-hour surgery.
The girls' mother, Bhumchu Zangmo, was both nervous and happy about the operation to separate the twins finally taking place, said Elizabeth Lodge, chief executive of the Children First Foundation that's assisting the family.
She will spend Friday praying and meditating. "But we just did not know what we would find".
"We keep making guesses as to how long this will take, but the reality is until the operation starts and ultimately we get to see what is connecting the girls, we won't really know how long", said Joe Crameri, Melbourne Royal Children's Hospital's head of pediatric surgery.
"She still has this extraordinary calmness about her, which is just wonderful".
Nima and Dawa were born by caesarean section on July 13 a year ago in a regional hospital in the Himalayan Kingdom of Bhutan, shocking doctors and the twins' family who had been expecting twins, but did not know they were conjoined.
Dr Sherub first met the girls when they were only a day old and played a major role in getting the twins to Australia, having already spent time in the country as the victor of a medical scholarship. A few of the muscles on her limbs are not developed, because they have not been used.
The doctor leading the team of 18 surgeons, nurses and anesthetists said the operation possibly could go into the night.