Performance reports released under the Official Information Act show St John met half of those contracted response time targets in 2010, but in particular failed to reach the urban targets, which have not been achieved since 2007.
"Reasons for this include demand for ambulance services rising faster than funded service capacity, increasing congestion on urban roads and new housing developments resulting in wider areas to be covered."
The report to Health Minister Tony Ryall in March said there had been a consistent growing demand for emergency services of around 4 per cent each year. This was expected to rise at a similar rate because of population growth, changing social habits (including greater alcohol-related problems), an ageing population and more patients with chronic illnesses.
The growing demand is most notable in Auckland and Christchurch, where new housing developments mean ambulance stations have to cover larger areas which, in turn, leads to an increased "job cycle time" - the time it takes to respond to a 111 call, deliver the patient and be available for the next job.
Traffic congestion and lack of road etiquette in cities also slows response times, says the Ministry of Health report, and damage to Christchurch roads after the February earthquake made the problem worse.
An extra $1 million was allocated to "address the response time issue" in urban areas, as well as to recognise the increased demand pressures created by the Rugby World Cup.
The report also mentions the 100 additional fulltime-equivalent ambulance officers funded and recruited in 2009 who were placed in rural and remote areas under most pressure, leading to improved response times.
St John chief executive Jaimes Wood said the key issue in main urban areas in meeting response time targets was the increasing demand on services and discussions were continuing about resourcing, particularly in Auckland and Christchurch.
"Our national workload continues to increase each year between 4 per cent and 6 per cent and one needs to appreciate that a city like Auckland grows at the equivalent of a Dunedin-sized city each year."
Mr Wood said operational funding to deliver services was subsidised by St John by $15 million a year, excluding the significant operational facilities and volunteer inputs which combined were valued at in excess of $50 million a year.
He said additional ambulance officers funded and recruited by all ambulance providers in 2009/10 were targeted at double crewing, not at improving response times, and St John did not receive all the 100 extra officers.
