The separate escapes of two mental health patients from the hospital responsible for the most complex psychiatric cases in western Sydney points to an inability to provide longer-term care when “demand is almost insatiable”, medical sources say.

A man charged with murder after a stabbing attack in Merrylands on Tuesday had allegedly absconded from Cumberland hospital. In an unrelated matter, it has been alleged that another of the hospital’s patients caused a car crash that killed two people on Saturday.

The New South Wales premier, Chris Minns, on Thursday said the growing pressure on the public mental health system needed to be addressed, including by implementing recommendations from the Bondi Junction stabbings inquest.

He said he was concerned about the security failing that led to patients to abscond and that “there needs to be a full investigation into the circumstances relating to these patients”.

“We need to make sure that forensic hospitals, psychiatric wards have the kind of security that’s in place to keep the public safe,” Minns said.

His comments came a year after psychiatrists threatened to resign en masse due to working conditions in the public system and the risks of providing substandard care to patients.

One mental health professional, who asked not to be identified, told Guardian Australia that two psychiatrists at Cumberland hospital quit last year. They were replaced by doctors who were contracted to work fewer hours.

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However, the doctor warned that full staffing may not have prevented this week’s incidents.

“If anything helps, it’s having the resources to do more work with patients to address long-term trauma and mental health issues,” they said.

“The focus at the hospital is on symptom control, with not enough time or resources for longer term care and therapy.”

They said “demand is almost insatiable” at Cumberland – a referral hospital for western Sydney patients with the most complex mental health needs – limiting the impact of the doctors’ work. Meanwhile, Westmead, Mt Druitt, and Auburn hospitals have “almost zero capacity” for inpatient mental health.

‘It’s a systemic issue’

The Cumberland branch president of the NSW Nurses and Midwives Association, Nick Howson, on Thursday said there was a “systemic issue that’s been brewing over a number of governments both Liberal and Labor”.

“The system is broken. We’re just waiting for it to hit the ground, we’re still falling,” Howson said.

“It’s not the fault of Cumberland, it’s not the fault of Western Sydney local health district, it’s not even the fault of NSW Health, it’s a systemic issue.”

A spokesperson from the Western Sydney Local Health District said it is “deeply concerned by the tragic incidents” and that the matters were before the courts.

“A formal review will be undertaken and will include the care and treatment of the patients alleged to be involved, and security protocols at Cumberland hospital. The review will include an external senior psychiatrist,” the spokesperson said.

“NSW police are notified when a patient absconds from mental health care, as they were in both of these cases.”

Minns acknowledged “the pressure on [the mental health] system is growing”.

While he welcomed increasing number of people seeking help when they needed it, he said: “We’ve got to have the capacity there to accept them when they arrive. That’s the thing I’m looking at the moment.”

“The vast majority of those cases end up with the community safe, the individual getting the help they need,” he said. “This one has gone badly wrong and we need to make sure we’re learning the lessons from it.”