The CPSU/CSA said it has "serious concerns" following another incident at Casuarina prison's Unit 18 on Wednesday afternoon.

Unit 18 is a stand alone unit accommodating children transferred from Banksia Hill Detention Centre.

One Thursday morning the union said it understood multiple people have had to be taken to hospital, including three officers.

"We understand a nurse was assaulted and at least one detainee has also had to be taken to hospital," the CPSU/CSA said in a statement.

The union noted that this is "at least" the third attack in which Youth Custodial Officers have been seriously assaulted or harmed at Unit 18 in the past two weeks.

A Department of Justice spokesperson told National Indigenous Times there were two distinct incidents on Wednesday.

"A Youth Custodial Officer was injured in an incident at Unit 18 involving a detainee yesterday morning and received medical treatment offsite. They were not taken offsite in an ambulance and the officer's arm was not broken," they said.

"In a separate incident yesterday morning, staff responded to a detainee causing cell damage at Unit 18. The detainee had set paper and books alight and officers were able to quickly extinguish the small fire.

"Three Youth Custodial Officers escorted the detainee to hospital where the detainee was checked for smoke inhalation. The detainee was returned to the Unit the same day. No officers required treatment offsite for smoke inhalation."

Branch Assistant Secretary of the CPSU/CSA Melanie Bray said multiple Youth Custodial Officers working at Banksia Hill or Unit 18 "have gone to work and left in an ambulance" over the past fortnight.

"Others have narrowly escaped serious physical injury. The effects of these traumatic incidents continue to compound with every incident that occurs," she said.

The union described the "continued threat" to safety at Unit 18 as "a dark shadow" over the successful ballot of members, which accepted the Department's offer for a replacement agreement on Wednesday afternoon.

The new agreement includes a commitment to the Adaptive Operational and Staffing Dynamics for Banksia Hill Detention Centre, which sets a baseline of measures to begin addressing the serious workplace health and safety concerns held by members at the Centre. However, the adaptive regime does not currently apply to workers or incidents at Unit 18.

Ms Bray said whilst this was an important first step for workplace health and safety at Banksia Hill Detention Centre, the union still holds "grave concerns" for the welfare of members, particularly at this time at Casuarina Unit 18.

"Members have made significant wins for baseline safety measures over the past year and these incidents continue to highlight how incredibly unsafe these working environments are and how important it is to make safety a priority. We urge the Department of Justice to continue to work with our members to continue to develop and implement safe solutions," she said.

"Safety must be the first and last consideration at both youth custodial estates. However, our members are still telling us that when they speak up about safety concerns they are being met with resistance and that cannot continue. Our members will continue the fight for their safety and the safe running of these centres.

"Whilst we recognise that these environments are incredibly complex with, at times, competing legal obligations, every single action must have workplace health and safety as the absolute priority. It cannot be a matter of foregoing safety to meet other requirements, the costs are too high to both children detained and our members, as the continuing events sadly demonstrate. Safety first, safety is the only way forward."

This follows a major incident at Unit 18 on Saturday.

Indigenous children are drastically over-represented in Western Australia's justice system.

Multiple experts have previously said conditions in youth detention in WA are unhealthy and dangerous for children detained in the system.

The WA Department of Justice spokesperson said that while there have been staff injuries during a number of recent incidents, there have been no occasions at Banksia Hill Detention Centre or Unit 18 in the past fortnight where Youth Custodial Officers have required to be escorted in an ambulance.

"Any assaults on staff by persons in custody is unacceptable. The Department of Justice is continuing to assess operational security in youth custodial facilities to enhance the safety of staff and detainees at the sites," they said.