A staff member at a youth justice facility suffered moderate injuries following an assault by a young offender.

The incident followed an attempted assault of another staffer in July at Te Puna Wai ō Tuhinapo, near Rolleston, Canterbury.

The National Union of Public Employees (NUPE) says the incidents are "serious", adding that the union has been "continually raising concerns about risk and staffing numbers".

In response to questions from RNZ, Oranga Tamariki deputy chief executive of youth justice services and residential care Dean Winter confirmed there were two incidents at Te Puna Wai in July.

"In the first incident a young person allegedly attempted to assault a staff member. This incident was promptly and safely de-escalated by staff, and the young person subsequently arrested by police.

"In the second incident, a staff member was allegedly assaulted by a young person, sustaining moderate injuries.

"Again, this incident was quickly de-escalated ... and the young person was subsequently arrested."

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A staff member was also injured while attempting to intervene with a young person potentially causing harm to another.

"This incident was deemed to be accidental, and the staff member has been provided support."

NUPE union secretary Janice Gemmell told RNZ the incidents were "preventable" and a "real concern".

"The incidents are serious - staff are suffering serious head injuries and other injuries and it is unacceptable.

"NUPE has been aware of ongoing harm to staff and has raised concern and solutions to the highest levels."

Gemmell said there were a series of concerns being raised by members, including concentrations of young people with high and complex needs, low staffing and risk of ongoing harm to staff and young people.

"NUPE has been continually raising concerns about risk and staffing numbers. We do not want to see members seriously hurt or killed at work."

Winter said the safety and wellbeing of everyone at the youth justice residences was an "absolute priority" for Oranga Tamariki.

"We know there is a significant level of risk within our residences, and our leadership teams at Te Puna Wai and across Oranga Tamariki work hard to maintain a safe operating environment for our staff and young people."

Oranga Tamariki regularly reviewed operating capacities within the residences.

"From time to time, we have staffing shortfalls due to unplanned absences or other reasons, and these are managed through casual staff and our operational leaders filling in as needed on the floor."

Winter said all instances of harm or injury were taken "extremely seriously".

"If an incident does occur, we have clearly understood review processes to identify how we can prevent it from happening again and we have reporting channels in place for any possible incidents, concerns or feedback.

"We share the concerns about the incident at Te Puna Wai, and we are making sure our staff member and their colleagues are well supported."

The agency had a "significant change programme" under way, which included improvements to rostering, training to support staff and upgrades to infrastructure.