The 58-year-old was caught with child sexual abuse material, including photos and videos, on a USB and on devices seized from his home by police in September last year.
His vile behaviour was only caught because he left a USB containing child sexual abuse material behind when he sold his house.
The horrified new owners alerted authorities after plugging the USB into their computer and finding the repulsive material, the court heard.
The former Marngoneet Correctional Centre general manager – who lost his job when the charges came to light – would have a more difficult time in prison than "any other prisoner" because of his position, his lawyer Sally Flynn said.
"He needs to be treated differently to other offenders who come before Your Honour," she told judge Patricia Liddell.
Prisoners could "attempt to stand over him in an attempt to gain knowledge" on how to traffic drugs into prison or on security details, she said.
She said he would likely have to be kept in protection for the duration of his sentence.
She said Harper had experienced a "spectacular fall from grace" from his senior role and position as breadwinner of his family.
His wife had been a stay-at-home mum but had to start a cleaning business to make ends meet, the court heard.
"Their family are in some financial peril due to Mr Harper's actions," Flynn said.
The court heard Harper was involved in managing sex offenders through his job.
Judge Riddell said it was "hard to fathom" that a prison general manager would not have considered how his actions could affect victims.
She said it was "concerning" he appeared to have a "refusal to confront himself" about why he downloaded, watched and shared the material.
"This man is not facing up to reality when it comes to his motivations."
Marngoneet Correctional Centre is a 559-bed medium security prison at Lara near Geelong, adjacent to maximum security HM Prison Barwon.
Harper will be sentenced in November.
