The two unions representing mental health nurses at the new $1 billion hospital say adolescent inpatients have attacked several nurses this year. Some have allegedly been kicked and punched and attacked with four inch nails, the Australian Nursing Federation says.

The nurses are concerned that staffing levels are not being increased sufficiently to safely manage the planned opening of four new beds in the unit this month. The expansion will take the ward from 12 to 16 beds.

Acting secretary of the Victorian branch of the ANF Paul Gilbert said although the hospital had agreed to boost nurse numbers during the day, it was refusing to increase those rostered on to the night shift from three to four.

He said if the hospital started opening the new beds on Thursday without an extra nurse rostered on to the night shift, some nurses would exercise their right under the Fair Work Act not to work because they believed it posed an unacceptable risk to their health and safety.

"The likelihood is that those nurses will exercise their right not to go to that ward for their shift," he said.

Mr Gilbert said hospital management had told him patient assaults on nurses had increased since the ward opened earlier this year. He did not have figures, but had heard anecdotal reports of assaults.

"We welcome the expansion of this much needed mental health service for children and teenagers, but extra patients require extra nurses, particularly in such a complex health environment," he said.

Assistant Secretary of the Health and Community Services Union Paul Healey said he had heard of about nine assaults on nurses at the hospital's mental health unit this year and said his members were also concerned about safe staffing levels. However, he said his members were happy to continue negotiating with the hospital rather than walk out on Thursday.

Mr Gilbert said the nurses' concerns were exacerbated by the design of the new ward which separated patients into various areas, creating more opportunities for nurses and patients to be left alone.

Executive director of nursing at the hospital Bernadette Twomey would not answer questions about how disruptive the action would be or how many assaults had occurred but said the four beds would open gradually so the hospital could monitor the impact before changing the nursing roster.

"It's important to note additional nurses are always made available on this ward in response to increased patient acuity and that won't change. We are committed to ensuring the safety of all members of staff, as well as patients, wherever they are in the hospital," she said.

jmedew@theage.com.au