Record-breaking numbers of people are being treated at Christchurch Hospital including a spike in children with the highly contagious Respiratory Syncytial Virus.
The flux has lead to an urgent change to hospital visits with a ban on children seeing patients.
The hospital's emergency department experienced a "massive increase" in patients with up to 70 more a day than expected.
Along with children suffering from RSV and other "very unwell people", staff are dealing with more injuries from skiing and icy conditions.
The situation has been mirrored across the country where hospitals are postponing surgeries and creating extra bed space for children as they deal with a sharp surge in RSV.
On Friday, 22 children were in intensive care or high dependency units with RSV across 11 of the country's 20 district health boards.
Wellington Regional Hospital had 26 children in the wards with RSV and respiratory-type illnesses who did not need to be in intensive care, while Hutt Hospital had 13 last week.
Yesterday a Starship Children's Hospital spokesperson confirmed 12 children were in its paediatric intensive care unit with RSV.
Institute of Environmental Science and Research data showed weekly visits to the six main hospitals for RSV has more than doubled in the past week, from 204 to 538 presentations.
Only 34 cases were recorded between April to September last year.
The situation has led to the Canterbury District Health Board changing visiting rules for Christchurch and Burwood Hospitals.
Patients can only have one visitor at a time and no children.
"Fewer people equals less risk to patients," the DHB said.
"Please wear your own face covering… don't visit if you are unwell."
CDHB chief executive officer Peter Bramley said "record-breaking numbers" of people were assessed and treated" over the weekend.
"Some of the presentations were children whose parents were concerned about RSV… with some children requiring admission to hospital.
"Everyone has seen a massive increase in the numbers of very unwell people they are seeing.
"Urgent care clinics have been seeing similar numbers of people to Christchurch Hospital's Emergency Department, where there's been a sharp rise in the numbers of adults and children coming in with respiratory issues."
On Saturday the Emergency Department treated 390 people - a record for the hospital.
The 24-Hour Surgery assessed and treated 373 people on Saturday and a further 403 on Sunday.
"Records were also being broken at Riccarton Clinic on Sunday where they saw 177 patients in 12 hours - 78 of them were children under 6 years old," said Bramley.
"This is more than double the number of children we would expect to see… Moorhouse
Medical was also busy with significant numbers of children with respiratory illnesses."
He commended staff who worked over the weekend and had advice for those who were sick and considering medical care.
"While most people seeking health care after hours and over the weekend needed to be seen by a clinician, it's timely to remind everyone that you can phone your own family doctor after hours for free health advice," he said.
"Simply call your general practice team's number after hours, follow the instructions to be put through to a nurse who can advise you on what to do and where to go if you need to be seen."
Despite a winter surge that's swamped hospitals, respiratory syncytial virus (RSV) isn't at all new to New Zealand.
In fact, it's so common that most children have been infected with the virus by age 2.
Known to cause infections of the lungs and respiratory tract, RSV's symptoms are usually mild and typically mimic a common cold, but they cause a severe infection in babies - especially premature infants - and the elderly or those with weak immune systems.
New Zealand's Health Navigator website lists specific symptoms such as a runny nose, coughing, sneezing, fever, wheezing and not feeling like eating – and these usually appear in stages and not all at once.
In very young infants with RSV, the only symptoms may be irritability, less activity and breathing difficulties – but the virus can also cause serious illness, including bronchiolitis and pneumonia.
Like many other respiratory illnesses, Māori and Pasifika children are disproportionately affected, with three to five times as many admissions as other groups.
People of any age can get another RSV infection, but infections later in life are generally less serious.
People at highest risk for severe symptoms include premature babies, young children with congenital heart or chronic lung disease, young children and adults with compromised immune systems, as well as older adults, especially those with underlying heart or lung disease.
RSV - no cure but help is available
There's currently no readily-available vaccine for RSV - although scientists are exploring one - nor is there any specific treatment for it.
But most RSV infections go away on their own in one to two weeks, and parents can help their children by ensuring they rest, drink plenty of fluids and use a humidifier to increase air moisture.
Symptoms can also be treated with medicines such as painkillers, nose drops or sprays, cough syrups and drops, throat lozenges and decongestants.
But parents are advised to talk to their healthcare provider before giving their children cold and cough medicines, as some contain ingredients that are inappropriate for children.
For infants and children who are at high risk of severe illness, such as babies born prematurely or with congenital heart disease or chronic lung disease, a medicine called palivizumab, and given by injection, can prevent severe RSV illness.
But it can't help cure or treat children already suffering from serious RSV, and it can't prevent infection.
"The best way to prevent spread of the infection is to always practise good respiratory hygiene, especially people with symptoms that might be due to RSV," Huang said.