Doctors are calling for urgent access to a new immunisation to protect babies from the dangerous RSV virus.

Three states in Australia have just agreed to fund nirsevimab to protect babies against the respiratory virus that has hospitalised hundreds in New Zealand in recent winters.

The disease could leave seriously affected infants struggling to breathe, eat and drink, with many needing oxygen and some ending up on ventilators.

Infectious diseases paediatrician Emma Best is leading the call to give at-risk New Zealand under-one-year-olds access to the medicine.

"That's those babies born premature and with other medical conditions where RSV becomes more than just a respiratory illness but something that leads them to needing intensive care," she says.

The immunisation is not a vaccine, but a monoclonal antibody.

It's given as an injection and protected for only a few months but could get vulnerable babies safely through their first winter, Dr Best says.

Without it, the virus is likely to remain the highest reason for babies to be hospitalised each winter.

The disease is very distressing, including for parents, she says.

Babies are often so busy trying to breathe, they can't eat or drink, she says.

"We don't have any medicines to make them better from the virus. We just support them with oxygen, sometimes even ventilation in intensive care."

Trials overseas show the nirsevimab could cut hospitalisation rates by 80 per cent.

There's a sticking point to getting it funded this year - it's not yet approved for use in New Zealand.

It's been given the go ahead by the United States, Europe and Australia last year, with Queensland, Western Australia and New South Wales funding it this winter to various degrees.

Pharmaceutical company Sanofi manufactures the immunisation under the brand name Beyfortus.

A spokesperson says it has started a conversation with Pharmac but is not able to speculate on its future availability.

Pharmac has been approached for comment.

-Rowan Quinn/RNZ.