A report released on Monday found police had carried out interviews with vulnerable people after their lawyers said their clients did not consent.
It followed an investigation by the state's police watchdog, the Law Enforcement Conduct Commission.
Chief commissioner Peter Johnson SC said police had failed to address problematic interviewing practices despite issues having been raised by the courts.
"This is a systemic problem and it is the responsibility of the NSW Police Force to give clear and accessible guidance to officers about how to properly approach the interviewing of vulnerable people in custody," he said.
"The system where telephone advice is given to young persons by the Aboriginal Legal Service or Legal Aid is of fundamental importance to the criminal justice system in this state."
In some cases, police had asked interviewees to confirm they had exercised their right to silence before proceeding to ask further questions about the alleged offence.
In other instances, records were not made by custody managers after a vulnerable person's lawyer indicated that their client wanted to exercise their right to silence.
The report stemmed from a complaint about the arrest and interviewing of a 14-year-old Aboriginal boy in northern NSW.
The commission made 19 recommendations to improve police interviewing, training for custody managers, using body-worn video and the creation of guidance for officers.
26°C