Auckland City Hospital is asking people to stay away unless it's a serious emergency as it struggles to cope with record demand.
Dr Barry Snow, director of adult medical services at the hospital, said the adult hospital was experiencing occupancy levels in the high 90 per cents.
"The number of patients presenting to us is running around 220 per day consistently, when normally it would be around 180-190, and we have more than the usual number of winter flus."
Auckland DHB's chief medical officer Dr Margaret Wilsher said people needing immediate hospital attention will get the treatment they need and the same high level of care.
"However, we ask that you only visit our emergency department if you need to."
Doctors in the district and around the country have seen a significant increase in consultations for flu-like illnesses in the last week and were now above the seasonal threshold level, according to ESR (the Institute of Environmental Science and Research).
Auckland DHB had the third highest consultation rate with 83 cases (146.2 per 100,000 people). Whanganui and Waitemata had even higher consultation rates for their populations with five cases (301.2 per 100 000 people) and 37 cases (292.7 per 100 000 people) respectively.
Flu infections were most common in children aged 0 to 4, schoolkids and people of Asian ethnicity.
Wilsher said Auckland City Hospital's emergency department was experiencing the "busiest period in history" and it has seen the highest number of patients ever during a four-week period.
"Our target is to see people within six hours of arrival, and we prioritise which patients are seen first based on the seriousness of their conditions," she added.
"We are requesting the public's help to ensure hospital services are available for those who really need them."
The hospital is planning for higher capacity levels at this time of year, but is struggling to cope with the current demand - which is above the usual winter increases.
To free up bed space for those who need them, patients will also be discharged as soon as they are well enough to go home.
Waitemata DHB's North Shore and Waitakere hospitals which service the north and west of the wider Auckland region had seen smaller increases - about 2 per cent and 16 per cent respectively - in patient numbers come to their emergency departments in the last month compared to the same time last year.
Waitakere Hospital ED's busiest day during the last month was on July 2 when 213 patients turned up.
A spokesperson for Capital and Coast DHB said its emergency department patient volumes was about the same as this time last year - with around 170 people coming in each day.
The Herald reported on July 6 that Middlemore Hospital was also being hit with record demand for care across all hospital services.
Waikato Hospital was in "overload" on May 23 and had to reschedule surgeries and turn away non-emergencies after an influx of patients in its emergency department.
How you can help:
• Seeing your GP for all non-urgent health concerns.
• Visiting your local accident and medical clinic for minor illnesses and injuries - you can find an appropriate GP or accident and medical clinic by visiting Healthpoint.
• If you're unsure where you should go, call Healthline for FREE advice from a nurse on 0800 611 116. Interpreters are available 24/7.
• If it's a serious or life-threatening emergency, don't hesitate to go to the emergency department, or call 111.