One Illawarra ambulance station was left unmanned, and others were "drastically understaffed" on Tuesday night, with paramedics concerned resources will remain depleted over the busy Easter period.
Health Services Union Ambulance Division state delegate Tess Oxley said the situation was "entirely unacceptable" and called on NSW Ambulance to immediately boost staffing levels.
"We were notified by our members in the Illawarra and Shoalhaven on Tuesday night that the area had been left drastically understaffed," she said.
"Wollongong was operating with no general duties car, just an intensive care crew. And that was the only intensive care resource from Helensburgh to Kiama.
"Other stations were operating on depleted crews or with only one officer on duty to respond. And, alarmingly, Warrawong was left completely unmanned so that suburb, and surrounding suburbs, were left without an ambulance resource last night.
"So the public have been placed at unnecessary risk, and paramedics have been exposed to increased fatigue in an area already facing high demand."
Ms Oxley said understaffing was also a safety issue, especially in light of a spate of assaults on paramedics in the region in recent weeks.
She said the union had notified NSW Ambulance earlier this year that there were 38 unoccupied positions in the Illawarra and Shoalhaven.
"It is our understanding these positions are yet to be filled by NSW Ambulance, showing complete disregard, yet again to the people of this region," she said.
However a NSW Ambulance spokesperson said all paramedic rosters in the Illawarra were currently fully staffed.
"NSW Ambulance is currently finalising a number of additional roles to enhance the service in the Illawarra," the spokesperson said.
"This includes 24 permanent paramedic positions who will operate out of Dapto and Bulli this year. This is in addition to 12 staff already on the ground at Bay and Basin station as well as six staff at Berry, which opened earlier this year."
The spokesperson said a number of paramedics who were rostered to duty on Tuesday evening were unable to attend their rostered shifts due to "short notice unplanned absence".
"NSW Ambulance utilises a number of existing arrangements to ensure short term vacancies can be filled where possible. During the afternoon and evening two Sydney crews assisted with coverage and workload as well as local on-call paramedic coverage."
Ms Oxley said while the additional paramedics set to be added to the roster were a "good start", it still wasn't enough to fill vacancies and cater for rising demand for ambulance services.
"If they are unable to maintain the essential levels currently, there is a concern that these additional rosters will also be left unfilled and the Illawarra/ Shoalhaven left uncovered yet again," she said.
"Even if all unoccupied positions were filled, and we get the staffing enhancement, we'd still need at least 40 additional paramedics on top of that.
"That's because the Illawarra Shoalhaven region has a rapidly increasing population; and there's a rise in chronic illness so the scope of paramedics has expanded - we don't just deal with emergencies but pre-hospital care.
"So lengthy delays at hospital emergency departments and lack of bulk billing at GPs all cause people to rely more on ambulance services."
This Easter, a traditionally busy time of year for paramedics, Ms Oxley urged people to consider whether their urgency was an emergency.
"Call Triple Zero if you feel like it's an emergency," she said. "But if there's an alternate pathway, like visiting a GP, please consider it."