Mental health nurses at three Sydney hospitals say they're unable to keep patients safe from COVID-19 without further support.

This comes after 26 patients in the Nepean Hospital’s mental health unit contracted COVID-19.

In a statement, NSW Health said the priority at the mental health wards at Nepean and Cumberland has been to protect staff, patients and to provide care.

Former national mental health commissioner Professor Ian Hickie says people with mental illness are at particular risk of COVID-19. He believes immediate recruitment in the private sector would have helped to take pressure off emergency departments.