Conjoined twins Nima and Dawa have undergone the first in a series of tests to determine when they will be fit for delicate separation surgery.

The 14-month-old girls from Bhutan, who are joined by the torso, spent today having various tests and scans at Melbourne's Royal Children's Hospital to give doctors an idea of when the marathon operation can proceed.

"We need to just double check everything is alright generally," anaesthetist Ian McKenzie said.

The twins underwent several tests at Melbourne's Royal Children's Hospital today. (9NEWS)

"Once we get the results of all that, we'll be very keen to make a date and plan for surgery in the future. So far, so good."

The girls underwent a four-hour MRI scan with the results to be reviewed in the coming days, before medical experts finalise their treatment plan early next week.

The surgery, estimated to cost $250,000, will be paid for by the Victorian government. (Courtesy: Herald Sun)

Mr McKenzie said the sisters would have "quite a lot of tests", because doctors had scant information before their arrival in Melbourne on Tuesday.

Nutrition levels will be an important guide, with the girls' condition meaning they cannot be weighed separately.

"They're pretty skinny, they haven't been able to practise crawling the way normal kids would, so maybe their muscles haven't developed so well but we want to just double check everything is alright, generally," Mr McKenzie said.

"There's a whole lot of things they might have got in Bhutan they wouldn't get in Australia, and we need to just have our guard up that we haven't got a surprise illness we hadn't thought of, that might be very common there."

Dr Joe Crameri said this morning he remains positive about the outcome of the surgery. (Nine)

The planned separation at the Royal Children's Hospital includes splitting the girls' shared liver.

The hospital's head paediatric surgeon Joe Crameri is reassured the twins are active and interacting with one another and that mum has been feeding the girls well.

The family arrived in Australia on Tuesday. (Nine)

An exact operating date is yet to be locked in, with the test results gathered today likely to decide when the twins will be ready for theatre. (Nine)

"They look like happy healthy girls who are reacting quite well with one another," he said.

"So far, everything is going along the pathway that we've been hoping for."

Dr Crameri said image scans will be key to determining if it's the right time for the girls to separate.

The surgery, estimated to cost $250,000, will now paid for by Victorian taxpayers, with any extra money raised being put towards after-surgery costs for the girls.

If the surgery goes ahead and is successful, it will be months before the girls are well enough to travel home to Bhutan. (Courtesy: Herald Sun)

Nearly $95,000 has been raised so far by the Australian public after a public plea for donations. After-surgery expenses are estimated to cost about $50,000 for each girl.

The babies are believed to be Bhutan's first recorded conjoined twins.

They were born by caesarean section on July 13 last year in a remote area of the Himalayas.

The girls are joined at the lower chest and through their stomachs. (Supplied)

The identical twin girls were joined at the thoraco-abdominal region, which relates to the thorax and the abdomen.

If the surgery goes ahead and is successful, it will be months before the girls are well enough to travel home to Bhutan.

With AAP