Two infants are in ICU in Wellington Hospital following a peak in respiratory illnesses in recent days.
Around 100 children have been admitted to Wellington Regional Hospital with RSV (Respiratory syncytial virus) over the past four weeks, with 20 children currently in hospital.
New Zealand Royal College of GPs medical director Dr Bryan Betty said he had seen an upswing in respiratory illnesses in children in his own Porirua practice.
"We saw none of it last year mainly due to the lockdown of the borders," he said.
"This year it's re-emerged and it's caused quite a bit of pressure on both general practice and hospitals."
"We had none of these illnesses in the country last year because of lockdown - children's immunity has dropped a little bit, and the illness they're actually getting is slightly more severe than what we would normally see."
A common winter condition, RSV causes inflammation of the lungs and respiratory tract. Betty said it could be severe in children without immunity, causing some to be admitted to ICU in very rare cases.
"There's certainly been a lot of admissions and presentations to emergency departments, urgent aftercare facilities and being admitted onto wards in hospitals."
"Potentially some children can end up in ICU, it's something that can happen in rare instances."
A letter sent home from one Wellington daycare centre also warned that parents be alert to symptoms of influenza-like illnesses, such as a cough, fever or sore throat.
Early Childhood Council Chief Executive Peter Reynolds said he was not yet aware of the recent spike being more concerning than usual.
"Every year various viruses and bugs will hit childcare centres, sometimes it happens in specific geographic centres and sometimes it's more general."
While the practices had been in place pre-Covid, the pandemic had made it easier for centres to quickly manage infection.
"There's a pile of health and safety issues that centres are responsible for," he said.
"Within that there are very clear rules that if a child is unwell, they get isolated at the centre, the parent gets called and the child goes home. And it's exactly the same with staff."
"But by that stage they already were infectious and it's been passed on to a number of other children - that tends to be how these things happen."
Especially due to the sensitivity around Covid, centres would be engaging in "hyper-sanitation" this winter, he said.
"All those processes we would continue to see centres operating under, whether there's an RSV virus running around or not."
Capital & Coast and Hutt Valley DHBs has been reporting a high volume of presentations to the emergency department of Wellington's Hospitals, creating a backlog of patients.
CCDHB Child Health clinical lead Dr Andrew Marshall said admissions of children with respiratory illnesses had seen a peak over recent days.
Hutt ED had seen around 116 children with respiratory and winter-type illnesses this month, with 24 requiring admission to hospital.
He advised that everyone be mindful of the symptoms of respiratory illnesses.
"Adults and older children get milder RSV illness but can pass it on to babies so we strongly urge people to maintain good hand hygiene, stay home if they have any symptoms of a respiratory tract illness, keep children home from daycare or kindergarten if they have coughs and colds, and wear masks on public transport – and to contact Healthline on 0800 358 5453 or their GP for advice."