Alyssa Nolan leads prayers for conjoined twins Trishna and Krishna

After several hours of positioning tubes to ensure there was no pressure on the eyes, surgeons made their first cut about 10am.

Plastic surgeon Tony Holmes said everything was so far going to plan.

"At the moment most of the major (skin) flaps are already up and looking particularly healthy and the bone is exposed ... for virtually half the exposure we need," he said.

"We're ready now for the neurosurgeons who are starting up right this minute to start removing the bone on the back half of the head, which is the dangerous bit in that that's where the brain is still joined and there are a couple of blood vessels."

Kishore of Melbourne
He said the chance of the operation being a success remains the same at just 25 per cent.

There is a 50 per cent chance they will suffer brain damage and a 25 per cent chance one of the sisters will die.

"The unknown ... is what actually happens when you separate finally the cerebral circulations, because that is a change in hemodynamics (blood movement) so the pressures will be different in each twin," Dr Holmes said.

"It's over those few early minutes when the pressures equilibrate in the brain, that they're the things that we're worried about."

'I'll pray for you'

Earlier today, Alyssa Nolan had a heartfelt message for conjoined twins Trishna and Krishna as they entered their final separation surgery this morning:

"Good luck, and I'll pray for you at school on Monday," the bubbly eight-year-old said. "And I hope you get better soon."

Alyssa knows more than most what the two-year-olds are going through because she's already been through it. She and her twin sister, Bethany, made worldwide headlines in 2001 when surgeons performed a 20-hour emergency operation to separate them.