The sexual assaults occurred just days before mental health beds at the hospital were stretched to breaking point, and management took the unprecedented step of creating a virtual, pop-up ward for eight mentally ill patients.

The revelation follows Age reports on Wednesday about violent incidents at Dandenong Hospital and the Royal Children's Hospital - where a nurse's hair was pulled out and another was punched in the face.

Australian Nursing Federation Victorian branch assistant secretary Pip Carew said the attacks were symptomatic of stress on the hospital system.

''Our members are telling us that they have not witnessed this level of aggression and inappropriate behaviour before. We are calling for more beds to take the pressure off the system and nurses. When there is a delay in getting into the inpatient unit, there is potentially an escalation of occupational violence.''

She said many assaults on nurses were not reported. The nurses' union is also demanding to know why at least two nurses were recently attacked at the Royal Children's Hospital by an aggressive adolescent patient from a low-security ward. The patient allegedly punched one nurse in the face and ripped another's hair out during a scuffle in March, resulting in significant nerve damage to the nurse's scalp.

At Dandenong Hospital, one nurse was threatened with a knife last weekend and another had part of her breast bitten off and needed plastic surgery.

A psychiatric nurse at Monash Medical Centre said being threatened and assaulted was a part of his daily life.

The nurse, who was punched in the throat by a patient and did not want to be named, said the clinical process was geared towards making space for the next patient.

''We help society's most vulnerable and downtrodden during the most difficult time of their life. When people are unwell they can act really badly but the system is unwilling to deal with this because it is expensive.''

The nurse's union said the spate of attacks highlight the state government's failure to honour its pre-election promise of spending $21 million to make hospital staff safer.

The government has only committed $5.8 million over four years to improve training and install duress alarms in some wards.

A recent audit by the union revealed that there are 24 fewer mental health acute beds than when the Coalition came to power in 2010.

A government spokesman said the Coalition had budgeted a record $1.14 billion for mental health services this year and invested in more than 140 new beds in the past two years.

With Julia Medew