The officer had left the control room in the Westfield centre in Sydney's Bondi Junction for a toilet break when Joel Cauchi, 40, launched his  stabbing spree at Westfield Bondi Junction in April 2024.
He killed six people and injured 10 others over three minutes before being shot dead by NSW Police Inspector Amy Scott.
The security officer returned to the control room and only spoke to a triple-zero operator 10 minutes after the incident occurred, an inquest into the tragedy was told on Monday.
In audio of that call, the guard - who cannot be legally identified - said there was an armed offender within the facility and that shots had been fired.
"Do you know if anyone's been injured there?" the operator asks.
"We're not aware," the guard replied.
She then changed her answer, saying three to four people had been injured and two people stabbed.
At the NSW Coroners Court on Monday, international counterterrorism, policing and security expert Scott Wilson called this response "inadequate".
"It was just all mixed up and it should have been a lot clearer," he said.
"Ten minutes in is just not good enough."
He said the call may have suggested that both a gun and a knife had been used by the attacker.
Mr Wilson said he could not believe the security guard was not aware of the extent of the casualties by the time she made the call.
"She must know she's dealing with multiple victims of a stabbing," he said.
Giving unclear information to the triple-zero operator could have led to confusion among attending police operators, the court was told.
While at the time, police knew that Cauchi was the sole armed offender who had been shot, information could have come in from the triple-zero call about another potential offender with a gun, Mr Wilson said.
He also questioned why the security guard remained in the room instead of being replaced by someone more competent as the emergency and its aftermath unfolded.
This was despite obvious deficiencies in her knowledge being flagged prior to the attacks. Further training had been scheduled.
"If you've not got faith in her, why leave her in that room?" Mr Wilson asked.
He acknowledged changes in procedures made after the incident by Scentre Group, which runs the Bondi Junction Westfield.
These include that two people have to remain in the control room at all times, and that officers do not need to get authorisation from their superiors before calling triple zero in an emergency.
Cauchi was experiencing psychotic symptoms at the time of his stabbing spree after being diagnosed with schizophrenia as a teen.
He had been successfully treated with antipsychotics for decades before his private psychiatrist formed a plan to wean him off the medication.
By mid-2019, he was not taking any more medication, and by early 2020 he had stopped seeing a psychiatrist regularly after moving from Toowoomba to Brisbane.
The inquest continues.
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