Chief Medical Officer Dr Margaret Wilsher said the hospital regretted it had to postpone five out of the 51 general surgery patients due Wednesday.
The situation for Thursday was unclear.
It was reviewed twice a day in light of the number of patients admitted through the emergency department overnight.
At this time of the year, hospitals experience a rise in patients with respiratory illness and chest infections.
However, at Middlemore Hospital, spokeswoman Lauren Young reported: "It is an unprecedented demand and our staff are under pressure."
"On Monday, we had 388 people through the accident and emergency department, which is a new record.
"We are asking people not to let themselves get too sick before they seek help and to go to their doctors early. And, it's not too late to get the flu vaccine."
Last Thursday, Middlemore was running at up to 10 per cent above normal capacity and yesterday [Wednesday] was 2 per cent over.
Elective surgery is done at Manukau Surgery Centre, off the main campus at Middlemore, which keeps the main campus free for acute surgery and means cancellations are uncommon.
Waitakere and North Shore Hospitals were managing elective surgery demand without postponements, said Waitemata District Health Board spokesman Matt Rogers.
"But our occupancy is very high - we are getting close to 100 per cent."
Waikato Hospital is very busy, running at 96 per cent of capacity but this might increase.
Since the beginning of July it has had a small number of cancellations due to inpatient capacity, said spokeswoman Kathryn Jenkin.
These were no greater than any other year.
In Northland, Whangarei Hospital was busy but had not had to cancel elective surgical procedures.
Alternatives for the sick
• Visit Healthpoint to find a GP in your area.
• Healthline nurses can assess patients over the phone. Call free on landline or mobile 0800 611 116
• For non-urgent, less serious health concerns when your doctor is closed, contact one of the out-of-hours Accident and Emergency centres.
• For serious injury or serious health concerns go to the hospital Emergency Department or ring 111.
• If your child is unwell, it's always best to seek medical attention sooner, rather than later. Don't wait to see if their condition gets worse.