Conjoined Melbourne twins undergoing separation surgery today

Conjoined Bhutanese twins Nima and Dawa have entered the operating theatre to undergo life-changing separation surgery in Melbourne.

A team of 18 surgeons, nurses and anaesthetists at the Royal Children's Hospital will work for more than six hours to separate the 15-month-old girls, who are joined at the torso and share a liver.

Head of paediatric surgery Dr Joe Crameri, who is leading the team, says the delicate procedure could go 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," he told reporters on Friday.

"It is unlikely that we will have completed the whole procedure by 4pm this afternoon."

Dr Crameri is optimistic but expects some challenges to arise, as it is unclear what parts of the girls are connected.

"I feel quietly confident that we are going to find something we can deal with in a very straightforward manner, that's what I'm hoping," he said.

"But there are challenges with these types of operations and I think we've got the team around us to deal with those."

Among Dr Crameri's concerns is the possibility the sisters share a bowel.

"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," he said.

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

One of the other risks was the use of anaesthetic, as doctors are unsure how one twin will react to the other receiving it.

The operation had previously been postponed after last-minute checks revealed the sisters were not ready.

The sisters were brought to Australia with their mother Bhumchu Zangmo in October and have been staying at the Children First Foundation retreat in Kilmore.

The charity's chief executive Elizabeth Lodge said Ms Zangmo was both excited and nervous about the surgery.

"This morning she used the word happy, yesterday she said she was a little bit scared and just pleased that today is finally here," she told 3AW radio.

"She believes in these surgeons and Dr Karma (Sherbub) from Bhutan has arrived and he'll be with the girls in theatre which is also very reassuring for mum."

The procedure 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 twin's rehabilitation and return home.