The three patients suffered heart failure while waiting for paramedics despite having time-critical injuries.

Ninety-year-old Norma Jones was forced to perform CPR on her son, Brad, for 30 minutes as he lay choking at their Blackheath home last month.

"It was a horrific half hour," Brad Jones' brother-in-law, Rolley Clarke, told Fairfax. "She has heart troubles; it was incredible she didn't go into arrhythmia".

Paramedics tried to resuscitate the 54-year-old local hospital wardsman but he was pronounced dead after a subsequent heart attack.

The ambulance service has blamed surges in demand for the delays.

But senior ambulance sources say it reflects the decreasing availability of ambulances, which has played out in two other recent fatal cases in a four-week period in October-November.

An ambulance called for a patient in Hornsby suffering breathing problems in late October arrived after 25 minutes. By the time the patient reached hospital they no longer registered a heart beat.

Late last month it took 30 minutes for an ambulance to reach a man suffering chest pains in the south-western suburb of Minto Heights.

A single paramedic was dispatched from Harrington Park about 15 kilometres away with backup travelling more than 40 kilometres from Warragamba.

Nearby ambulances were delayed in hospitals at Campbelltown, Liverpool and Bankstown, a source said.

Health Minister Jillian Skinner said the Ambulance Service had hired 205 new paramedics since 2011 and responded, on average, to emergency cases within 7.5 minutes.

"I offer my sincere condolences to the families of these patients and assure them these incidents are being thoroughly investigated," Mrs Skinner said.

She said 80 per cent of patients were transferred to hospital emergency departments within 30 minutes.

The closest ambulance available at the time of Mrs Jones' 7.30pm call was in Springwood, more than 40 kilometres away.

Local paramedics on a day-shift were sent to cover shortages in Penrith that morning and did not return until 9pm. A single night-shift paramedic was busy with another case before eventually being able to respond.

The president of the Australian Paramedics' Association, Wayne Flint, said paramedics were often left to wonder whether patients may have survived if they were able to arrive sooner.

"It's time for the NSW government to take responsibility for these recurring events and commit cash and [paramedics]."

The paramedics' union said delays in emergency departments were stretching resources.

Under NSW regulations paramedics must remain with patients in the emergency department until a bed becomes available.

The number of ambulances "lost" each day due to hospital delays has risen to nearly 20 from six in 2005, the Auditor-General found. This means that they are unavailable or taken off while waiting for hospitals to take their patients, known among paramedics as "trolley blocking".

Australian Paramedics' Association Secretary Gary Wilson said: "Paramedics say they are being used to prop up the health system in 'peak' periods that are becoming regular. But an emergency system should be surge proof".

The union said last week that hospital delays caused the first ambulance called for cricketer Phillip Hughes to take 23 minutes to reach the Sydney Cricket Ground.

Yearly demand for ambulance transport is growing at 4 per cent, or about 20,000 cases.

According to the Productivity Commission, Sydney's ambulance response time is the longest for any capital city.

A confidential 2010 report by the Ambulance Service projected that without more funding paramedics would reach 340 heart attack patients within the eight-minute benchmark next year, compared with 1000 in 2008.

"Significantly more people are likely to die … unless immediate action is taken," the confidential analysis reported by Fairfax Media found.

The head of the Ambulance Service, Ray Creen, said he was "always regretful of any delay" and was investigating the three cases.

"A surge in demand is often unpredictable," he said.