And there are fears evidence may now not be admissable in court.
It has emerged the nurse was not registered with the Nursing Bord, An Bord Altranais, meaning that the examination of individuals and evidence collected may potentially be compromised.
The HSE was aware that the nurse was not registered and has launched an investigation into how she was able to continue to practice.
Nurses are required to be registered with An Bord Altranais to practice here but the nurse in question was removed from the register in 2009 for non payment of fees.
The HSE says it was notified at the time and that the information was made available within the organisation but that it did not filter down to the Sexual Assault Treatment Unit in Letterkenny General Hospital.
It became aware of the issue in the last few weeks after a review at the Treatment Unit and says the nurse in question is no longer on duty.
Twenty-five people who were examined at the unit in the last 18 months were contacted yesterday by gardai and the HSE.
The Rape Crisis Network of Ireland says they were told that forensic evidence gathered may not be admissible in court.
Gardaí say the matter has been brought to the attention of the Director of Public Prosecutions.
The HSE says it has appointed two independent experts to carry out a review to establish how a staff member could continue to practice without being registered.