Maternity units at NSW hospitals were so overstretched in the run up to Christmas that six women were transferred to Newcastle on a single day.

Some neonatologists say the high number of transfers is symptomatic of a system that has no "surge capacity", and also  means   women being hurried out of hospital before they are ready, to make room for new arrivals.

Mothers who deliver their babies at Liverpool Hospital are sometimes forced to wait for beds in the maternity unit to be vacated by patients overflowing from the emergency department, they say.

"They don't actually quarantine beds for the mothers," one doctor said.

Thirty-one pregnant women were transferred to John Hunter Hospital in Newcastle in December, prompting concerns about the state's ability to cope with the increasing number of premature deliveries.

Most transfers happen when a woman has a complication or prematurity risk that cannot be adequately handled at her local hospital, and there are an estimated 1350 in NSW every year.

It is safer to transfer premature babies to hospitals with neonatal intensive-care units in utero than to wait until they are born.

Pregnancy and Newborn Services Network adviser Daniel Challis said a hotline that was set up in 2013 for clinicians to get high-level advice and ensure all transfers were strictly necessary took 320 calls in the first 12 months.

"On occasion the capacity of individual intensive care or high-dependency units can be reached, which may necessitate the transfer of women or babies out of their local network or further from home than is ideal," Dr Challis said.

John Hunter is the main obstetric unit in northern NSW for women who have complications with their pregnancy and whose babies may need to be cared for in a neonatal intensive-care unit.

It delivers about 4000 babies a year and 25 women are transferred there each month on average.

Newcastle doctors have requested the government to build nearby accommodation for women with high-risk pregnancies, who can be confined for weeks, so they do not take up hospital beds.

The high demand for neonatal beds has caused tensions among medical specialties.

Obstetricians would like to prioritise beds for women with high-risk pregnancies who are about to deliver, but neonatologists are reluctant to move out premature babies who have been long-term residents at the hospital.

The general rule is last in, first out.

But John Hunter director of obstetrics Henry Murray said this was unsafe.

"We believe when a baby is stable we should be able to send it to a lower-level neonatal unit to allow the higher-risk ones to remain in the hospital," Dr Murray said.

t"Unfortunately, there has been resistance by the mothers, which is understandable, but we should be giving resources to the babies that need it the most."

Mothers should be given written information at the time of their admission advising that they may need to transfer to another hospital if their bed were required, he said.

Maternal beds could also be freed up by creating a better system for midwives to look after babies at home and by lowering the caesarean rate, Dr Murray said.

The problems have been compounded by a baby boom in the ACT, which also diverts patients to NSW in times of high demand.

The ACT recorded 4999 births in 2013-14 – its highest number on record and an increase of 70 per cent since 2001-02.

An ACT Health spokeswoman said women were encouraged to go home within 24 hours of giving birth, with daily visits from a midwife for the first 14 days.

The NSW government funded three additional neonatal intensive-care cots in July 2014 and plans to add 11 intensive-care cots to John Hunter Hospital by 2017.

Labor MP Andrew McDonald, a paediatrician, said neonatal cots were a continuing problem.

"They run them at bare necessity, which gives them no surge capacity," Dr McDonald said.

"I don't think there would be any neonatal paediatrician in NSW who didn't want more beds. They're constantly juggling beds."

But Royal Australian and New Zealand College of Obstetricians and Gynaecologists vice-president Vijay Roach said there was an adequate number of intensive-care beds.

"The neonatal intensive-care bed is the most expensive bed in the hospital," Dr Roach said.

"You can't have them sitting around empty."

NSW Health Minister Jillian Skinner said the overwhelming majority of women delivered in the hospital of their choice.

"'In a limited number of cases, women transfer to another hospital to ensure both the mother and baby receive the best possible care in the most appropriate setting," she said.

The local health district responsible for Liverpool Hospital declined to comment on emergency patients occupying beds in the maternity ward.