"Accessing the right service at the right time for the right condition is an ongoing pressure and challenge," he said.
The service had done several things to cope with the growing demand, he said.
"We are working with community groups to establish a deployment point in the eastern Wanganui area. This would provide a second St John site to respond from during the daytime and would help decrease our response times.
"We are also trialling an additional patient-transfer ambulance working in Wanganui to manage inter-hospital transfers and other such services."
The two Wanganui ambulances were fully crewed during the daytime and fully crewed 86 per cent of the time at night, Mr May said.
"There are challenges for St John as demand increases. This brings opportunities however, and St John is very focused on trying to provide the best possible patient pathways and outcomes.
"An example is the work of health shuttles to provide alternative transport opportunities for patients to maintain and gain access to healthcare services, which ultimately is good for the patient, their health and preventing more urgent ailments developing."
New Zealand Ambulance Association chairman Mark Quin said a huge challenge lay ahead to cope with demand.
"In some respects, I think we're just putting our finger on the dam at the moment," he said. "We're putting in all these initiatives but I don't know if we're reacting quick enough.
"The elderly population is growing dramatically every year so I can't see demand stopping any time soon."
The service increasingly needed to be a "jack of all trades" and this was also putting pressure on the system, he said.
"Often we're the default [healthcare provider] - especially in weekends and after hours.
"In some cases, people are unable to access a GP so they call an ambulance. Others might have an appointment in three days but want to be seen sooner so they call an ambulance, even though the problem's only minor.
"Another increasingly big area of or work is mental health. It's another area that lacks services, particularly after hours and during weekends."
St John had put more crews on in response to demand - but more resources would only ease the problem to an extent, he said.
"We're meant to be responding to emergencies."
Further Government support wouldn't go astray, however, he said.