SYDNEY, Nov. 9 (Xinhua) -- Bhutanese conjoined twins, Nima and Dawa, entered an Australian operating room on Friday for a life-changing surgery -- with a team of 18 professionals expected to work for more than six hours to separate the girls.

However head of paediatric surgery at Melbourne's Royal Children's Hospital, Dr Joe Crameri who will perform the surgery, said that the nature of the 15-month-old girls' connection is not fully clear and the operation may continue into the night.

"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," Crameri said.

Nima and Dawa, who are joined at the torso, travelled to Australia with their mother in October thanks to efforts by a children charity group.

While it is known that the girls share a liver, preliminary scans are unclear as to whether they also share a bowel, which may complicate the procedure.

"We know the bowel is mixed and it could be entirely separate and sitting next to one another or it also can be that the girls share the bowel and we have to find a way of dividing that," Crameri said.

"The one benefit we all have is we are all born with a lot of bowel and you can afford to decrease that."

The girls' mother is said to be anxious but coping well and is comforted by the presence of a Bhutanese paediatric surgeon who flew to Australia to assist with the operation.

The Victorian State government have offered to pay the hospital bill, expected to total at least 250,000 U.S. dollars.