"This is the highest and fastest rate of increase in demand for our services we have ever experienced," Thornton added.
People are being urged to see their family doctor in the first instance, and encouraged to ring the free Healthline on 0800 611116 for advice before heading to hospital.
"We are asking everyone to help keep our emergency department free for people with life threatening illnesses and injuries," said Thornton.
These include those experiencing chest pain, head injuries, severe blood loss or major accident.
Middlemore Hospital is the busiest of all hospital emergency departments in New Zealand, but Thornton said she was aware that other hospitals were also facing similar pressure with the onset of winter.
In the past four weeks, the hospital had seen 802 more patients compared to the same time last year. More than half were children.