Potentially life-threatening "category one" jobs are supposed to have paramedics on-scene within 10 minutes, but Fairfax Media has found NSW Ambulance is often not able to assign an ambulance to begin travelling to the job within that time.

In just one week in July, data obtained by Fairfax Media details five such jobs, which in one case meant a woman with what had been described as a traumatic injury did not have a paramedic arrive until nearly 40 minutes after a triple zero call was made.

Another involved a baby who had fallen from a table and was having seizures, although the Ambulance Service denied having records of this case.

The NSW opposition has called for an independent inquiry into the resourcing of the Ambulance Service, amid increasing response times.

A productivity commission report has found Sydney's average response times when 90 per cent of cases are counted are some of the highest in the country, while the proportion of qualified paramedics employed is below states such as Victoria and Queensland.

Health Services Union secretary Gerard Hayes said the under-resourcing of the service was reaching dangerous levels.

"Lights and sirens cases are reserved for life threatening cases only," he said. "Unfortunately we are seeing daily examples of patients waiting in excess of 10 minutes for an ambulance to become available, let alone arrive".

Last Friday Channel 7 reported an elderly woman waited more than two hours for an ambulance, lying on the street after a fall. Health Minister Jillian Skinner apologised to the woman and has asked for an investigation into what happened.

But Mr Hayes said she was in "damage control".

"What paramedics are calling for is real solutions to the problem, not another inquiry," he said. "Until there is an increase of at least 250 paramedics in Sydney alone the public will remain at risk."

Labor health spokesman Walt Secord​ said the government needed to stop blaming patients and paramedics and should launch a fully independent inquiry into resourcing.

"Each day there is a new case of a patient failing to receive immediate care," he said. "The last thing we want is the NSW Ambulance Service bureaucrats investigating themselves."

HSU councillor and paramedic Greg Bruce said the category 1C cases identified by Fairfax Media would include people in severe pain, or with breathing problems or suspected heart attack or stroke.

"Five to 10  or 20 minutes can mean the difference between having damage to the heart muscle or not, because it hasn't had that extra time not getting oxygen to it," he said. "People are likely in pain and it's also very distressing for the families.

"Yet you will hear [radio reports of inadequate resources] three to five times a day during the busy periods, sometimes up to 10 times a day."

NSW Ambulance confirmed that the average response time for category one cases had increased to 11 minutes and  27 seconds in 2014/15, compared to 10 minutes 47 seconds the year before.

She said category 1C was the third most serious case (behind 1A and 1B). The patient who had not been met by a paramedic for 40 minutes involved "a 76-year-old woman who had hip pain", but the first ambulance assigned to her had been re-routed to a more serious case.

"NSW Ambulance is fully resourced," she said. "A problem facing NSW Ambulance is that resources are being tied up responding to incidents which could be better dealt with by people utilising other health pathways."

She said the service took a call every 28 seconds, but that up to 40 per cent of people who received an ambulance, a paramedic and a stretcher did not need one.

Health minister Jillian Skinner said the HSU was running a campaign and it along with "outrageous" Labor party's attacks were undermining the work of paramedics and triple zero call takers.

"In 2014/15, response time performance for life-threatening cases (classified 1a) was 7.65 minutes across NSW, against a target of 10 minutes," she said. "NSW Ambulance should be praised, not attacked."

She said since the Liberals had come to government the Ambulance NSW budget had been increased by more than 26 per cent.

"An additional 270 full- time equivalent paramedics have been employed since 2011," she said. "And we are expanding the capacity of NSW Ambulance by building the first major new ambulance stations in almost 20 years in metropolitan Sydney."

She said she had demanded an explanation of the Channel 7 case from Ambulance NSW chief executive David Dutton, and was expecting an answer Monday afternoon.