Research carried out by the Guardian into new Nauru documents shows that self-harm and sexual abuse incident reports were routinely altered and downgraded in seriousness by the security company tasked with protecting asylum seekers on the island.

Save the Children, one of the major agencies in the detention centre, protested that they were “usually downgraded without any clear justification”.

A spokeswoman for Wilson Security said the company followed the reporting guidelines established by Australia’s immigration department and denied that incident reports are systematically downgraded in breach of the guidelines.

The Guardian’s investigation into the leaked files reveals that more than 100 reports made by other Australian companies working in the detention centre were downgraded in seriousness by Wilson Security.

Incident reports written by guards, caseworkers and medical officers at Nauru detention centre are reportedly classified in seriousness as “critical”, “major” or “minor”. The investigation shows that some are labelled simply as “information” reports.

The downgrading by Wilson was recognised by other providers, and apparently admitted by Wilson, as revealed in an email dated 6 September 2015 by a senior Save the Children’s manager on Nauru.

In one case an incident report of a self-harm threat by a child was made by a Save the Children worker in January 2015. The asylum seeker child had stopped taking her medication but told the worker “she will die and she doesn’t care about anything anymore”.

A spokesperson for the immigration department said a mechanism for reclassification was entirely appropriate.

The Nauru files are the largest cache of leaked documents published about the Australian immigration detention system.