Detective Senior Constable Michael Simonsen made the admission on Thursday during Zev "Velvel" Serebryanski's committal hearing in the Melbourne Magistrates Court.
Serebryanski, 60, is accused of sexually assaulting Mr Waks, 48, at the ultra-Orthodox Yeshiva Centre in Melbourne's southeast between January 1987 and December 1988.
Manny Waks is an advocate for victims of child sexual abuse in Jewish communities. (Mal Fairclough/AAP PHOTOS)
Serebryanski was extradited from the United States in April 2023 to face three charges of indecent assault and one charge of sexual penetration of a child aged over 10 but under 16.
Mr Waks, an advocate for victims of child sexual abuse in Jewish communities, of has given AAP permission to identify him as the alleged victim.
Sen Const Simonsen told the court he took a police statement from Mr Waks in February 2012 to clarify parts of the first statement he made to officers in September 1996.
Under questioning from Serebryanski's barrister Ian Hill KC, the detective admitted he did not recall asking the identity of the person Mr Waks claimed was present during one of the alleged assaults.
He also conceded he never attended the Yeshiva Centre to take photographs or obtain a floor plan, nor did he identify and speak to people who attended the centre at the time of the alleged abuse.
"I failed to do that," Sen Const Simonsen told the court.
An investigation into allegations against Zev Serebryanski (left) was opened and closed for years. (Diego Fedele/AAP PHOTOS)
The detective said the investigation was repeatedly opened and closed in the seven years he was the police informant as other witnesses came forward.
One of those witnesses was Yerachmiel Gorelik, who gave evidence via video link from the US on Thursday.
He told the court he was classmates at Yeshiva College with Mr Waks and aged about 11 or 12 when he was told about the alleged abuse.
Mr Gorelik could not be sure of the exact date of the conversation but he admitted telling other students the allegations.
The court was told Mr Gorelik emailed police in January 2012 to say he was reluctant to make a statement because there were gaps in his memory.
He ultimately made a sworn statement to officers in July 2013.
The committal hearing continues.
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