Almost a third of people heading to a NSW hospital emergency department don't start treatment on time amid the busiest conditions ever, new data shows.

NSW hospital emergency departments have never been busier, with an earlier-than-usual flu season coinciding more people waiting longer for treatment.

The latest official hospital emergency data released on Wednesday shows ED attendances topped 756,000 in the first three months of 2019 - almost six per cent higher than the same quarter last year.

Among the hospitals feeling the pressure was the state's $600 million, 488-bed Northern Beaches Hospital, where two out of five ED patients (40.3 per cent) didn't start treatment on time.

Patients waited even longer at the top-tier Westmead Hospital, where 47.5 per cent were treated on time.

Statewide, the average for on-time treatment dropped 4.3 per cent to 71.9 per cent.

The Bureau of Health Information quarterly report also showed the median time spent in ED swelled to almost three hours while one in 10 patients waited more than seven-and-a-half hours to begin treatment.

NSW Health said the early flu season was already being felt as more patients headed to ED with respiratory illnesses, fever and infections.

"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."

Ambulance responses also grew in the busy quarter, with paramedics making more than 3400 responses a day - 10 per cent higher than the same quarter last year.

Despite this, the median response time for critically ill patients remained at seven-and-a-half minutes.

The elective surgery waiting lists now stand at 83,625, though just one in 28 patients don't receive surgery on time.