Nurses working 24-hour shifts at a major WA hospital demonstrates the risks facing staff and patients in the public health system amid ongoing staffing pressures, a union says.

Unplanned sick leave and an exceptionally high number of intensive care patients on Wednesday forced Perth's Fiona Stanley Hospital to call for staff to work an overnight shift.

Two ICU nursing management staff volunteered to stay and worked until Thursday morning in roles that did not involve direct patient care, the hospital's executive director Neil Doverty said on Friday.

"They had taken an extended break (and wanted) to support their colleagues and patients," he said.

But the Australian Nurses Federation wants to know why there were no backup nurses available, saying the pair had effectively worked a triple shift.

The union said the incident would not have happened if the hospital's staffing levels were under control.

"Nurses being forced to work triple shifts is an outrageous solution to a problem we shouldn't have," state secretary Janet Reah said.

"We need effective staff incentive, recruitment and retention strategies if WA is to overcome the obstacle of the lack of staff clearly being experienced."

Ms Reah patient care could suffer if nurses became fatigued while working overtime.

"Particularly in ICU where the sickest of the sick patients are cared for," she said.

Mr Doverty said ICU nursing was highly specialised and not all nurses could backfill these positions.

"All other options were considered to try to cover these shifts," he said.

"This was a case of two managerial staff choosing to assist in the face of unprecedented demand, which they did ... to ensure the continued safety of our patients."

He said frontline nurses are not allowed to work 24-hour shifts and the volunteers continued to have regular breaks through the night.