A child was sexually abused by a sex offender who should have been monitored by police and two people were unlawfully imprisoned as a result of widespread mismanagement of the New South Wales child protection register.

On Thursday the NSW Law Enforcement Conduct Commission released the findings of a long-running investigation into police handling of the state’s child sex offender register.

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The LECC investigation has revealed that the force has mismanaged the register for almost two decades, resulting in more than 700 incorrect decisions. In one case a man who should have been subject to police monitoring committed an act of indecency in front of a 10-year-old girl on a bus in 2015.

In two other cases, people were unlawfully imprisoned for more than a year because they were erroneously placed on the register.

The NSW child sex offender register includes people who have been convicted of sexual or violent offences against children, and people convicted of having child abuse material.

Since 2009 the number of people on the register has increased by 83%, from 2,376 to 4,344.

The LECC’s investigation revealed that “significant errors” in the handling of the register began as early as 2002. In the past 17 years, it found, 96 people have been incorrectly left off the register, while 485 have had their reporting period – the amount of time their names appear on the register – under-calculated.

It also found that although the force had been aware of problems since as far back as 2014, it waited two years to begin reviewing case files.

When it did, its own review found errors in 44% of the more than 5,000 cases investigated. Those included “incorrect decisions by the NSW Police Force about which persons should be included on the register, and incorrect decisions about how long persons were legally required to make reports of their personal information to police”, the LECC said.

Shockingly, the LECC found that the man who committed the act of indecency against the girl on a bus did so just a month after he was incorrectly removed from the register.

In 2003 the man, referred to in the report as “Mr AA114”, was convicted of two counts of aggravated indecent assault of a child. He was released on parole in 2006, but owing to mistakes made when calculating his reporting period – including the failure to take into account another offence in 1997 – he was removed from the register after eight years.

In November 2015 the man “boarded a bus and committed an act of indecency in front of a 10 year old girl”, the report reveals.

After serving 12 months in prison, the man was again placed on the register for 15 years until a review of his case by the LECC recommended he be subject to monitoring for the rest of his life.

In another case, a man who was convicted of two counts of using a carriage service to procure a child under the age of 16 for sex had his period on the register underestimated by seven years and nine months.

As a result, he was taken off the register in December 2014 when he should have been subject to police monitoring until September 2022.

In the more than 5,000 cases reviewed by police, 45 people were also found to have been have incorrectly placed on the register, the report states. Among those, the LECC found the two cases in which people were unlawfully imprisoned.

In one of the cases, a man known as Mr DD120 was placed on the register after a 2006 conviction for the indecent assault of a victim under the age of 10. Despite being a minor at the time of the offence, the man was incorrectly placed on the register and between 2010 and 2015 served 413 days in prison for failure to report to police.

In the other case, a man referred to as Mr NN was convicted of a sexual offence against an adult member of his family.

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“As the victim of that offence was not a child, he did not meet the definition of a registrable person under the [Child Protection (Offenders Registration) Act],” the LECC said. “Despite this in 2014 he was convicted of failing to comply with reporting obligations under the CPOR Act and received an 18 month good behaviour bond.”

According to the investigation, nicknamed Operation Tusket, the errors were the result of failures to correctly interpret rules surrounding the register, as well as insufficient resourcing amid the 83% increase in the number of people placed on the register in the last decade.

It also said that “almost since its inception” the multiagency system, in which police are supposed to be assisted in running the register by courts and government departments, “has not been functioning as parliament intended”.

The LECC said that since it began its investigation the state’s police had made changes to the register, including doubling the number of staff responsible for running it.

But the report makes 11 recommendations, including that the act of parliament governing the register be referred to the NSW Law Reform Commission for review.