Alan McBride, from the victims' group Wave, said although police had acted within the law at the time, there was a "moral obligation" to inform relatives.
Mr McBride urged police to contact all of the families affected as quickly as possible to allay fears.
The PSNI has apologised for the distress caused to families.
The body parts and tissue samples were retained as part of police investigations into suspicious and unexplained deaths between 1960 and 2005.
The cases, including 23 related to the Troubles, were revealed as part of a UK-wide audit of all police forces.
Mr McBride told BBC Radio Ulster's Evening Extra programme that although he personally did not know any of the 64 families whose loved ones' body parts had been retained, there was fear among many relatives that police would knock on their door.
"For these families to be told all these years later, when perhaps they have buried their loved ones and even gone through the grieving process and moved on - the potential here for re-traumatisation for some families I think is immense," he said.
Mr McBride called on the PSNI to ensure family liaison officers, or police with the "appropriate skillset" carried out personal visits to the relatives.
Meanwhile, the mother of a murdered schoolgirl - who had consented to the retention of part of her daughter's body as part of the police investigation - said it was very important that families were told the truth.
Margaret McAlorum's 16-year-old daughter, Megan, died from head injuries after she was assaulted in 2004.
Mrs McAlorum said the PSNI had kept her family informed "every step of the way" and had fully explained the need to retain the teenager's head as evidence.
The family later held a second burial for Megan.
Her mother said families would understand why body parts had to be kept in certain circumstances to help with investigations, but she said it would be much easier to cope if they were kept informed.
"If someone had rapped my door now, I would be extremely, extremely stressed," she said.
Assistant Chief Constable George Hamilton said the PSNI had acted within the law and that all the items had been retained for "police purposes", but he admitted relatives should have been informed.
He explained that until 2006 police were under no legal obligation to inform relatives.
Officers are now visiting affected families to advise them of their options.
ACC Hamilton said families "needs, wishes, trauma and upset" were at the heart of its response, but admitted that the matter could have been better handled.
"We offer an apology for the upset we are causing to families," he said.
"The problem has been that there was no review mechanism in place, there was no monitoring or audit so that decisions could be made... about how we would handle those pieces of human tissue, once the criminal justice element had been fulfilled.
He said police were satisfied that there was a "good reason" for retaining the items, a decision that was largely based on the judgement of pathologists.
ACC Hamilton said most of the samples will be suitable to be handed back or dealt with in accordance with family's wishes.
"Even if it does need to be retained, there still needs to be that important conversation with families to explain that we have these items and why we have it and what we intend to do with it," he said.
State Pathologist for Northern Ireland, Prof Jack Crane explained that material would be kept for two reasons.
"One would be that it would assist or help us determine the precise cause of death, and the other one would be for evidential purposes," he said.
"In many cases we carry out, we don't feel the need to retain material."
Prof Crane said some of these cases had not gone through the criminal justice system and could still possibly come to court.
"We would be failing in our duty if there was something that could possibly have evidential value and we had destroyed or disposed off that might still be required," he said.
"There are a number of old cases being examined by Historical Enquiries Team and on some occasions they have come to us to ask us what material we might have that may be of assistance to them."
Last week, it emerged that two police forces in England - Hampshire and City of London - kept body parts and tissue samples in 89 suspicious and unexplained death cases without notifying relatives.
The legislation was changed in 2006 making the retention of body tissue illegal.
There is fear among victims' families following the revelation that police kept body parts and tissue in 64 death investigations, a campaigner has said.