A 16-year-old Aboriginal boy is in a critical but stable condition in Sir Charles Gairdner Hospital after being found unresponsive in his cell at a Western Australian youth detention unit.

The boy is a detainee at Unit 18, the WA government's controversial youth unit located on the grounds of a maximum security adult prison, Casuarina.

On Thursday the state's Department of Justice said the boy was observed to be unresponsive in his cell during routine night checks around 2:00am.

The department said staff commenced resuscitation before the arrival of three paramedic teams.

Eaerly on Thursday the department said the teenager is at Sir Charles Gairdner Hospital in a critical but stable condition, and that all circumstances surrounding the incident are being investigated.

The department said detainees and staff at Unit 18 and Banksia Hill Detention Centre, the state's main youth detention facility, will be provided "counselling and supports".

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

Unit 18 has been the centre of controversy since it commenced operation in July last year.

In August 2022, statistics tabled in parliament revealed a shocking number of self-harm incidents and suicide attempts in Unit 18, as well as Banksia Hill.

In May this year, letters from detainees detailed abuse allegations at Unit 18 and Banksia Hill.

A class action involving hundreds of former youth detainees in WA is underway.

The Branch Secretary of WA's CPSU/CSA, the union that represents staff at the facility, Rikki Hendon said the union's "thoughts are with the family and friends of the young person involved in last night's incident".

"We wish the young person well in their recovery," she said.

"The union is supporting the psychosocial well-being of our members at Unit 18, particularly those present at the time of the incident. The union will not be making any further comments at this time."

More to come.