New data shows 43 per cent of those requiring almost-immediate care aren't seen on time in major Sydney hospitals.

Two in five people presenting to major Sydney hospitals with the second-most critical needs aren't being seen on time, official data shows.

The data released on Wednesday shows the delays are most apparent at Westmead Hospital, where patients who should be seen within minutes for injuries such as severe burns or severe breathing trouble, wait on average 16 minutes.

One in 10 category-two patients at Westmead wait 80 minutes or more for treatment - eight times the recommended maximum time.

The NSW Bureau of Health Information data shows at principal Sydney hospital between July and September, just 57.2 per cent of the 500,000-odd category-two patients were seen on time.

It wasn't much better for category-three patients - those with moderate blood loss or dehydration - with 38 per cent not seen within the recommended 30-minute time frame.

Across the state, 73.4 per cent, or almost three-quarters, were seen on time - a 2.9 per cent improvement on the previous year.

About the same amount spent less than four hours in emergency before being admitted to a ward or being sent home.

This coincided with a milder flu season and fewer presentations to emergency departments.

More than 717,700 people presented themselves to emergency departments between July and September, which is a 3.2 per cent decrease from the same period in 2017.

BHI chief executive Diane Watson says last year's severe flu season saw more patients at emergency departments compared with this year's milder flu season.

"While we need to take this year's milder flu season into consideration when looking at these results, it is still encouraging to see results at NSW level have improved compared with the same quarter last year," Dr Watson said in a statement.

NSW Health Deputy Secretary Susan Pearce said the improved on-time figures across the board was a testament to the efforts of doctors, nurses, paramedics and also the community, who vaccinated against flu in record numbers.

But Opposition health spokesman Walt Secord says the longer waiting times in some emergency departments shows the system is under pressure.