NSW Health said the early flu season was already being felt, as more patients headed to emergency and face longer than normal wait times.
A record number of people are presenting to NSW emergency departments and many are waiting longer for treatment - a situation doctors say is unsustainable.
The latest official data, released on Wednesday, shows there were more than 756,000 attendances at hospital emergency departments across the state in the first three months of 2019.
That was 42,247 - or almost 6 per cent - higher than during the same period in 2018 and the busiest quarter on record.
NSW Health said the early flu season was already being felt as more patients headed to emergency with respiratory illnesses, fever and infections.
The Bureau of Health Information report also the found median time patients spent in the ED jumped 12 minutes, while the number of people treated within clinically-recommended timeframes fell 4.3 per cent.
"At NSW level, the timeliness of care provided to patients has declined from last year," the bureau's chief executive Diane Watson said in a statement.
Ambulances were also kept busy during the quarter, with paramedics responding to more than 307,000 cases - 10 per cent higher than during the same period in 2018.
Despite this, the median response time for life-threatening cases remained at seven-and-a-half minutes.
The number of people on the waiting list for elective surgery stands at 83,625, up 8 per cent.
"These figures do not even show the full winter months," Deputy Secretary Susan Pearce said in a statement.
"While there is always room for improvement, it was pleasing to see that some of our hospitals improved their performance despite large increases in presentations and very unwell patients."
The Australian Medical Association's NSW president said the rate of growth in patient numbers in emergency departments was not sustainable.
"We have effectively seen one-tenth of the population of NSW go through our emergency departments over the first three months of this year," Kean-Seng Lim said in a statement.
NSW Labor labelled the figures "damning", particularly for the new Northern Beaches Hospital.
"There is no way Premier Gladys Berejiklian or Health Minister Brad Hazzard can claim after five months that these are just 'teething problems'," opposition health spokesman Walt Secord said in a statement.
"This clearly shows that the problems at the Northern Beaches Hospital are systemic."
Mr Secord last week successfully moved for a parliamentary inquiry into the operation and management of the hospital.