A woman has died after being stabbed in the neck in a mediation room at the Joondalup courthouse in Perth’s northern suburbs.

The woman aged in her 30s was rushed to Joondalup Health Campus after being attacked about 10:00am at the Joondalup Justice Complex on Reid Promenade.

She was treated at the emergency department but was “tragically” unable to be revived, police said.

“A 40-year-old man who is known to the victim is in police custody,” Acting Inspector Andrew Maher said.

“Major Crime Squad detectives are currently at the scene and are investigating the incident.”

Acting Inspector Maher said first aid attempts were made before the woman was taken to hospital.

“It would have been definitely horrifying … staff at the Joondalup Justice Complex will be assisted in any way with their welfare,” he said.

Police would not confirm why the woman was at the courthouse, or what weapon was used in the attack.

‘Get towels, get towels, she’s been stabbed’

Janet Lee was in a waiting area at the courthouse next to the mediation room when the woman was attacked.

“We heard a woman’s screams, and then a guy came running out shouting [for] security. It was all pandemonium then,” she said.

“The staff told us to go outside … when they came out, the staff were saying ‘get towels, get towels, she’s been stabbed’.

“And she’d been stabbed in the throat, in an artery, apparently.”

The Joondalup Justice Complex has been closed for the day and taped off by police. Photo: ABC

Ms Lee said before she left the courthouse she saw a man sitting on the floor surrounded by security officers.

“It was horrible … we walked away in shock,” she said.

Police and forensics officers remain at the courthouse, and the building has been cordoned off and closed.

“Following an incident at the Joondalup Justice Complex … the Joondalup Magistrates Court will be closed for the rest of today,” a spokeswoman for the Department of the Attorney General said.

“Matters listed for today have been adjourned and will be relisted.”

Police urged anyone who had witnessed the incident and had not spoken to detectives to contact them.