Police in New South Wales have for two decades maintained a secretive blacklist disproportionately made up of Indigenous children deemed to be at risk of committing crimes, who they target using “unreasonable, unjust and oppressive” tactics that in some cases are likely to be illegal.

Many of the children – including some as young as nine – have not been charged with any crimes and in some cases came to the attention of police because they were deemed to be at risk of domestic abuse.

On Thursday the Law Enforcement Conduct Commission – the state’s police watchdog – handed down the findings of its investigation into the Suspect Target Management Plan (STMP), an opaque system of “preventive policing” so closely guarded by NSW police that individuals are often not aware they are a target.

The two-year investigation found that while the STMP had been designed to monitor repeat offenders and prevent crime it often involved “intrusive” policing tactics, “unreasonable surveillance” and “harassment” that could in some cases increase young people’s risk of entering the justice system.

The LECC’s investigation reviewed the cases of the 429 children aged between nine and 17 who were placed on the plan between August 2016 and 2018.

The targets are disproportionately Indigenous. In some cases, Indigenous children were placed on an STMP despite never having been charged with an offence – including a nine-year-old from rural NSW who had been identified by police as a victim of domestic violence.

The nine-year-old, who had previously been “a person of interest” in other matters, was subsequently charged 94 times, the most serious being for aggravated robbery.

The NSW police commissioner, Mick Fuller, has previously said Aboriginal and Torres Strait Islander people made up about 55% of those – including adults – on an STMP.

But the LECC’s report found that figure could be substantially higher. Of the children examined during its investigation 72% were found to be “possibly” Indigenous on the basis of previous self-identification.

The NSW police disputed that figure, arguing only 47% of the cohort were Aboriginal because it “does not rely on self-identification figures” to calculate Indigeneity.

Instead it used an algorithm to determine the likelihood of Indigeneity if identification was “inconsistent over time”.

This was because, NSW police said, “there are many instances where a person accidentally (for example, a Pacific Islander thinking that ‘Islander’ equates to ‘Torres Strait Islander’) or maliciously, identifies as Indigenous in a police interaction”.

The principal legal officer of the NSW Aboriginal Legal Service, Nadine Miles, told the Guardian it was “hard not to draw the conclusion” the program was discriminatory “given the number of Indigenous children caught up in STMPs” and said it should not be used to target children.

“Our position is that the overwhelming detriment to individuals, communities and family units subject to this program outweighs the need for police to use such tactics when it comes to young children,” she said.

“We’ve got examples where families have been torn apart because of these tactics. We’ve seen children made to reside with other people to stop the harm being caused in the family units. It’s just not on and it’s appalling.”

The LECC said its investigation had found evidence young people were the subject of “coercive” policing practices and were four times more likely to be subject to “stop, search and detain” tactics compared with a positive strategy such as referral to a local PCYC.

“Monitoring STMP targets as recommended in the policy can have the effect of being intrusive and disruptive to a young person’s day-to-day existence,” the report said.

“These interactions do not appear to be diversionary in nature, and might serve to increase the likelihood that a child or young person will be charged and thereby drawn into the criminal justice system.”

Camilla Pandolfini, a senior solicitor at the Public Interest Advocacy Centre, co-authored a 2017 report that sparked the LECC’s investigation into the STMP. She said there were still “grave questions” about its use.

“Exposing these children to even more interactions with police and the criminal justice system has a serious negative impact on the young person and runs counter to the efforts that are being made to divert young people from the criminal justice system,” she said.

Among the controversial practices used by police were home visits, often in the early hours of the morning and late at night, which Pandolfini said were “likely to be unlawful”.

“We continue to have serious concerns about the lawfulness of these proactive policing measures, particularly seemingly arbitrary ‘home visits’,” she said.

“This policing practice has no lawful basis and should be ceased immediately.”

The LECC report said there were “considerations as to the lawfulness” of the policy.

The report described one case of a young person who was subjected to 195 home visits between November 2015 and March 2017, while in another police visited the home of a 16-year-old 10 times in a month, including once at 3am.

While the LECC did not make a finding on the disproportionate number of Aboriginal people subject to the STMP, it said it had “concerns” the basis for selecting STMP targets “does not demonstrate sufficient rigour” to prevent unfair targeting “of certain types of young offenders” and to “ameliorate officer bias”.

“For obvious reasons, even the appearance of discrimination in the application of a policy such as the STMP can have negative implications for its effectiveness,” the report said.

In a statement, the NSW police said it had accepted all 15 of the LECC’s recommendations and had committed to a “redesign” of the program which “takes a broader approach to recidivist and criminal behaviour”.