St Bede's College in Christchurch has refused RNZ's request, and emails suggest it has not responded to similar requests from the victim and police.
The complainant said a lay art teacher at St Bede's sexually abused him, starting when he was 15 years old, in the 1970s.
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A police investigation last year found the complainant "is a credible and believable witness and police believe that his complaint is genuine".
But the accused was unwell and not competent to stand trial, and there was little chance of success in any prosecution, the police said.
The school has also upheld the complaint.
RNZ requested a copy of the school's investigation under the Official Information Act, but St Bede's Proprietors Trust Board chairperson Shane O'Brien refused.
"The investigation report is not official information," Mr O'Brien said by email.
"The investigation was undertaken for and on behalf of, and is held solely as agent for, the Society of Mary (to which the complaint was made), which is not subject to the OIA."
There was also good reason to refuse for privacy reasons, Mr O'Brien said.
The complainant said the school also refused to let him see the report.
"One of my frustrations is a refusal to share results of the investigation conducted and how it was carried out," the man wrote to the Society of Mary in August this year.
"I think I am entitled to full disclosure in this respect."
"I spoke to the rector after your last email... he said that because the police investigation is closed he didn't see the point in police having it," a police detective wrote in October 2018.
"He said he would discuss with the order (Society of Mary) that owns the school. I haven't heard back as yet."
Documents showed the school offered the victim $7000 earlier this year, but he refused it.
"We do realise you may have been hoping for more, but if you were to consult... you would find that this amount is around what might be expected in New Zealand," Mr O'Brien's letter to the man said.
The man said he and two advocates met with the Bishop of Christchurch, Paul Martin, earlier this month to lay out what they saw as shortcomings in the school and church's handling of the complaint.
"The Bishop said he would look into this," the complainant said.
The Bishop and Society of Mary have been asked for comment.
In October, St Bede's College rejected an RNZ request regarding the investigation of another sex abuse complaint.
Christchurch man Peter Boock laid a complaint in 2011, after he learned a teacher at the school, Robin Pettit, was the man who had abused him decades before.
The school said it investigated at the time, but told RNZ that no investigation report existed.
Mr Pettit has admitted sexually assaulting Mr Boock in Dunedin in the late 1960s, but continued teaching at the college after the complaint and investigation.
Mr Boock has filed a police complaint, and the Teaching Council is also investigating.
