Fight for conjoined twins Trishna and Krishna

AT 8.30am on Monday, a Melbourne medical team will begin the most complex operation ever undertaken in Australia.

Fight for survival: Trishna and Krishna

AT 8.30am on Monday, a Melbourne medical team will begin the most complex operation ever undertaken in Australia.

If all goes according to their meticulous and well-rehearsed plans, the 16 surgeons, doctors and nurses hope Trishna and Krishna will be separated by about 9pm.

The surgeons will then continue to build the girls' heads and protect their precious brains, with the delicate work expected to continue until close to midnight.

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The groundbreaking bid to separate and save the two-year-olds will take place exactly two years and two days since the Bangladeshi orphans arrived in Australia, saved by Moira Kelly's Children First Foundation.

The Royal Children's Hospital team will have to combine many of the most complex individual operations they have performed into a single joint operation.

Having already performed four life-saving operations lasting up to 20 hours to split the sisters' shared blood vessels and sections of brain, neurosurgeon Wirginia Maixner is amazed at how far the girls have come.

But she is still acutely aware of the odds stacked against her team and their fragile patients.

There is a 25 per cent chance the sisters will die; a 50 per cent chance they will suffer catastrophic brain damage; and a 25 per cent chance her team will "pull it off and bring them through safely".

Ms Maixner said: "Realistically, I am actually more optimistic because they are in such a better condition and we have already done so much of what we need to do.

"But there are significant challenges ahead for the girls, you can't underestimate that.

"A lot of the things we do for these girls we do on a different scale for a lot of children.

"But the challenge of these girls is that nowhere have we had to put this all together in such a magnitude."

The first bridge will have to be crossed by two anaesthetic teams working simultaneously under doctors Ian McKenzie and Andrew Davidson, trying to safely balance the girls' individual sleep in spite of their shared blood flow.

"They (Trishna and Krishna) are in phenomenally good condition compared to when they came, and we are going to go in very optimistically," Dr McKenzie said.

The neurosurgeons will then take over, spending up to eight hours separating the final connections inside the girls' skulls - a thumbnail-size "bridge" of brain and two or three small veins. Plastic surgeon Andrew Greensmith and his team will then make a "Yin-Yang" shaped cut to split the skin between Trishna and Krishna's heads.

The cut is designed to leave large flaps of skin capable of growing hair that can later cover the girls' new skulls.

Silicone tissue expanders, which have been gradually filled with two litres of fluid in recent months to grow new skin - will then be removed.

The girls' skulls will then be cut, leaving exposed sections of brain the size of two dinner plates, and they will finally be separated.

Neurosurgeons may then need to add in synthetic brain lining to help seal their heads before the plastic surgeons can place plastic skulls on top of their brains, which are then covered by the skin.

"To make the operation a complete success and be compatible with survival, we need to have a nice watertight seal to the brain lining," Mr Greensmith said.

"It is not just a matter of separating and a simple closure, it is a significant plastic surgical reconstructive issue."

Ms Maixner said the decision was made almost two years ago that the team would never sacrifice one child to save the other.

"We considered both these girls have equal importance to life, and we either could save both girls and we would not risk one child for the other," she said.

After the surgical team is finished it will be up to the hospital's intensive specialists to care for the twins, who will remain unconscious at least until scans of their brains are taken on Wednesday, and possibly longer.

Their guardian, Moira Kelly, will also keep a loving eye over them, having accepted them as her daughters since they arrived here battling for life against a heart condition.

"Over two years have passed and Trishna and Krishna have had a huge impact on my life personally," Ms Kelly said.

"I now truly understand the term Mother Theresa lived by: 'Love until it hurts'."