Critically ill premature babies could be at more risk in the future if neonatal units are not expanded or new units built, an expert says.
Three of the country's five units are at near capacity and the situation will get worse as the population grows.
The neonatal units that hold level 3 - the most critically ill - babies are based at Starship Children's Hospital, Waikato Hospital, Wellington Hospital, Christchurch Hospital and Dunedin Hospital.
If the unit closest to the parents is full, babies are transferred to a unit elsewhere, and in extreme cases, to Australia.
Neonatal Trust chief executive Michael Meads said having babies and their parents transferred to a unit somewhere else in the country was a "huge upheaval".
In most cases the five units were enough to care for premature babies at the moment, he said.
"But I guess as our population grows, so do premature births, or at-risk births. It goes part in parcel. Eventually there will have to be more level 3 units.
"As time goes by it will only get worse and there will only be more demands put on hospitals to increase their capacity."
When medical emergencies happen to newborns, they and their parents sometimes have to be transferred to another part of the country quickly.
"All of a sudden they're picked up by a helicopter or raced out to an airport and end up in an entirely different region as to where they expected to have their baby. They've got no time to pack," Mr Meads said.
It could be terrifying for parents, especially first-time parents.
"Sometimes, when they do need to go to the other end of the country, it could be for months, and their life completely changes."
Waikato Hospital has been experiencing high numbers of critically ill premature babies. Its spokeswoman Mary Anne Gill said last week they were "over-capacity".
She said extra staffing had been brought in to care for the critically ill newborn babies born prematurely now requiring ongoing care.
"It tugs at the heartstrings really because these are parents who end up spending weeks, sometimes months, with their babies in the neo-natal intensive care unit."
By the numbers:
Starship Children's Hospital: 11 babies, 22 beds;
Waikato Hospital: 33 babies, 41 beds;
Wellington Hospital: 36 babies, 42 beds;
Christchurch Hospital: 38 babies, 40 beds; and
Dunedin Hospital: 8 babies, 16 beds.