More walk-outs by Sydney nurses are likely after staff at two major hospitals rallied over what they claim are chronic staffing shortages, while the NSW government concedes the health system is under pressure.

The Blacktown and Westmead hospital emergency services nurses staged the walk-out on Monday morning after completing their shifts to protest a lack of safe nurse-to-patient ratios and "severe understaffing" at the EDs.

The rallies - not classified as strike action - were part of a continued effort by western Sydney nurses to fix understaffing they say has led to 18-hour days and long shifts without breaks, worsened by the COVID-19 pandemic.

NSW nurses union general secretary Brett Holmes said with flu and COVID-19 presentations on the rise, the two EDs were regularly overwhelmed and "urgent measures" had not been taken by authorities to help combat the issues.

"Gauging the level of frustration and despair it's quite possible that other branches will look at this and consider whether they need to to also take similar action," Mr Holmes told AAP on Monday.

"There's no doubt that there's similar experiences and feelings across the rest of the health system, whether our members choose to follow the same pattern or do something different ... I just have to wait and see."

The NSW Nurses and Midwives' Association said 21 night duty nurses walked out at Blacktown hospital, while about 50 rallied at Westmead.

The demonstration comes after about 60 ICU nurses rallied at Westmead hospital over staffing shortages in February.

Premier Dominic Perrottet said flu season was putting extra strain on hospitals, admitting the health system nationwide was "under enormous pressure".

"It's been a very difficult time with COVID over the past few years ... But we've invested record amounts to get people through this difficult time," Mr Perrottet told reporters on Monday.

He said his administration was working closely with the federal government "to make sure we get better systems in place to put downward pressure on our public hospitals".

The state government has previously said it is aware of pressures on nurses and has met with the union to address their concerns.

NSW Labor leader Chris Minns said the Perrottet government had failed to adequately staff hospitals, leading to one in 10 patients leaving EDs because of delays.

"This has now led to this situation where we have a crisis on our hands in hospitals right across NSW, especially in western Sydney," Mr Minns said on Monday.

"This is something that the government has refused to talk about."

The government has faced a barrage of industrial action in recent months, including thousands of teachers walking off the job and train drivers refusing to use foreign-built rail stock, which has thrown the transport network into chaos.