Counties Manukau District Health Board spokeswoman Lauren Young said Middlemore Hospital was at 100 per cent capacity yesterday.
The ideal loading is 85 per cent, but Ms Young said being completely full "is normal for us".
"We do have quite a few respiratory medical cases."
Among other hospitals in the region yesterday, Waitakere was at 97 per cent of capacity, North Shore Hospital 95 per cent and Auckland City 93 per cent.
The clinical director of adults emergency medicine at Auckland City Hospital, Dr Tim Parke, said, "The emergency department has been very busy in the past two weeks with the expected spike in seasonal illnesses, particularly heart attacks amongst middle-aged people and severe pneumonia amongst the elderly."
A report on the Counties Manukau and Auckland DHBs' hospitals says they reported 267 cases of severe acute respiratory infections from April 30 to early June. Four people were admitted to an intensive care unit and one died.
The early winter spike in flu-like illness has been driven by increasing numbers of cases of the influenza strain A (H3N2), which can be particularly severe for the elderly and those with underlying medical conditions.
This year's influenza virus covers three strains, including H3N2.