Johnston said police had recovered memory cards from the backpacker lodge on which there were photographs of men who had been drugged and assaulted by Harris.
The memory cards revealed 24 different men - two of whom were still unidentified - who appeared to be asleep or only partly conscious. The images included pictures of the men with items of clothing apparently removed by Harris, and some in which their genitals were exposed.
Johnston said a forensic examination of the memory cards strongly suggested there were images of other victims that had been deleted.
The sequential recording of images on the memory cards by a numbered file name showed large numbers of images had been deleted, he said.
While forensic efforts had recovered some deleted data, there was still a large amount which had not been able to be salvaged.
"We know from the numbers of the photographs there are many, many images we can't recover. We are pretty confident we can say there will be [other victims]."
Harris' offending against the 24 victims at the centre of police inquiries was almost entirely confined to the three years he owned the Kaitaia hostel. There was one earlier identified victim dating back to 2005.
"We were greatly concerned about the number of victims," said Johnston. Police tracked down and individually identified 22 of the 24 men through a public appeal, working through the lodge guestbook and interviewing staff who remembered names of those who had stayed.
Johnston said it appeared that Harris had identified those he wanted to be victims of his crime some time before he struck.
Like one victim interviewed by the Herald, those targeted had moved from the backpacker part of the hostel into a spare room in Harris' adjoining home.
"The victims were specifically identified by Mr Harris and that's why they were asked to stay in that residence."
He said the police probe into the case came after two German backpackers became concerned there had been an "incident" - "they suspected something had happened".
The two young men went to Kaitaia Hospital the day of their concerns and got blood tests which then revealed drugs in their system that they had not taken.
"I'm really, really pleased they did. It would have been a hugely more difficult process without it."
Johnston said a complaint had been made to police the year before but the victim had been drugged six weeks prior and no evidence remained in the person's blood.
A police officer had spoken to Harris who denied any wrongdoing - yet didn't appear to have stopped preying on his guests.
He said Harris' crimes and those who were its victims was a particular focus for those investigating.
"Everyone involved was horrified at the thought these young people were leaving with this as their memory of New Zealand."
