An ambulance triple-0 call went unanswered for up to 20 minutes during a period of peak demand for emergency help earlier this year.
A source from the Sydney ambulance control centre said a triple-0 call on August 24 had taken 20 minutes to answer.
An Ambulance NSW spokeswoman did not deny this and said delays that day were caused by ''unprecedented'' demand over a two-hour period.
''NSW Ambulance experiences peaks and troughs of triple-0 calls. On August 24, NSW Ambulance experienced one of those peak days,'' the spokeswoman said.
''There were some delays encountered with no adverse patient outcomes.''
She said except for this two-hour peak, the centre answered 90 per cent of calls within 10 seconds.
Ambulance NSW denied reports from within the centre of a similar delay this month, saying they had no record of the incident.
But an Ambulance NSW employee said lengthy delays were becoming more common, as staff struggled to deal with call volumes.
''We just cannot cope any more,'' the source said.
Shortages, the source said, were acute on days when someone called in sick. ''If nobody wants to do overtime any more - which happens all the time because we are all sick of doing [it] - then we are just short,'' the source said.
The ambulance service receives a call once every 26 seconds - or more than 1 million each year - and between May and November this year it said 90 per cent of them were answered within six seconds.
But a review by the auditor-general this year found demand for ambulance transport had been growing at 4 per cent a year - or 22,000 ambulance trips annually.
The number of full-time staff in call centres had grown by about 7 per cent in the past five years, or by 14 positions to 238, according to data provided by Ambulance NSW.
''A crisis … is looming,'' said Wayne Flint, the president of the Emergency Medical Services Protection Association. ''A number of festering issues remain unresolved by ambulance management.''
Calls are first received at a triple-0 call centre and then patched through to an ambulance call-centre, which manages taking calls and dispatching ambulances.
The ambulance service said calls were routed between its four NSW control centres if not picked up within one second.
The opposition spokesman on health, Dr Andrew McDonald, called on the health minister to make call-waiting times public: ''This is life and death,'' he said.
Health Minister Jillian Skinner declined to comment.
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