Conjoined twins to undergo risky life-changing surgery in Melbourne this week

It's a case of third time lucky for conjoined twins Nima and Dawa who are expected to undergo marathon separation surgery in Melbourne on Friday.

After being twice delayed, the Bhutan-born girls are finally ready to undergo the six-hour procedure, head of paediatric surgery at the Royal Children's Hospital Joe Crameri told reporters today.

"Two false starts is not a good way to go but we feel reasonably confident we will be undertaking the procedure this Friday," he said.

"We want complete separation with two girls that are then totally independent so they are not sacrificing something to get that."

Dr Crameri said he'd caused confusion when he last spoke publicly of the surgery, as the girls' weren't healthy enough.

"Like any operation, we thought we were right to go surgically ... but the last minute checks with the girls showed they were not quite to the level we'd anticipated," he said.

"It was the right decision not to go ahead at that moment.

"We truly believed we were (also) at that point four weeks ago but last-minute testing timelines weren't right."

The 15-month-old twins, who arrived in Australia with their mother in October, are joined at the torso and share a liver.

It is also possible they also share a bowel, Dr Crameri said.

The girls' mum had been "obviously disappointed" when previous surgery plans were cancelled, he added.

"In a very Bhutanese way, she was very stoical about that and kept smiling.

"She is now excited."

The doctor hopes the surgery will be "straightforward" but admits if there is an issue, the twins are in the best place to get the care they need.

The surgery is expected to involve 18 medical staff, with two teams for the girls, plus nursing and anaesthetic support teams.

The sisters have put on at least 2kg since their arrival in Melbourne, under the care of the staff at the Children First foundation retreat in regional Victoria.

The surgery and recovery are expected to cost at least $350,000 and the state government has offered to pay the bill.

Other funds raised will go towards the twins' rehabilitation and return home.