Canberra: The Australian Border Force admitted internally that it failed to respond appropriately to allegations of sexual assault and abuse on Nauru but did not disclose these findings to a parliamentary inquiry.

A tranche of internal emails obtained by Guardian Australia under freedom of information laws reveals the reaction of the agency after the Guardian’s publication of the Nauru files. The agency deployed at least eight Australian Border Force officers to work on it, and contracted staff from KPMG and Hudson recruitment to conduct forensic work and data analysis. The internal emails show the Department of Immigration and Border Protection set up a special taskforce to assess whether incidents detailed in the Nauru files were appropriately responded to.

The taskforce’s final report identifies six incidents with a severity rating of “major” where the immediate action taken was not deemed appropriate. This includes two incidents of an alleged assault on a minor, and one incident involving an alleged sexual assault of a minor. There is also one incident of self-harm where immediate action was not taken.

These internal findings were not disclosed in the department’s written submission to a parliamentary inquiry, set up in response to the publication of the files.

“The department continues to assist and support service providers, the government of Nauru, and local Nauruan authorities to support continuous improvement to incident response and reporting practices, including referrals for additional services or to the Nauru Police Force in cases of possible criminal wrongdoing,” the department’s submission said.

“Of the 281 reports categorised as being major incidents ... 270 incidents had immediate and appropriate action taken ... 11 had insufficient information to determine what actions were taken and accordingly it is not possible to determine if appropriate action was or was not taken.”

The 11 incidents with insufficient information include two categorised as “concern for a minor” and two self-harm incidents.

The assessment also acknowledges issues with the classification of incident reports, saying “reporting of incidents in 2013 was poor with a number of incidents misclassified when compared with the incident management framework. This situation progressively improved over the following two years.”

Incident reports in the Nauru files are classified by severity as critical, major, and minor. There is also a large number of reports classified as “information”, or with no classification. However, analysis of incidents classified as “information” in the database, rather than minor, major or critical, shows many incidents that could be considered major or critical.

The internal emails identified 249 minor, unclassified and information incidents that did not have an appropriate response, and 106 where immediate action was not taken. Within these are more than 20 incidents that involve children, as well as family violence incidents. None of these figures was disclosed to the parliamentary inquiry.

The analysis also revealed that previous investigations commissioned by the department did not have complete sets of all incident reports about serious allegations on Nauru.

Previously the department has relied on reviews by the Child Protection Panel and by former integrity commissioner Philip Moss to assess the extent of serious incidents at the Nauru centre. But the Nauru files taskforce also revealed that six allegations of sexual assaults from their own files were not known to Moss, with a further two unknown.

The Child Protection Panel was not aware of five incidents relating to sexual assault, and it was unclear whether they were aware at all of a further 10.

A separate tranche of files also sets out emails from the secretary of the department, Michael Pezzullo. On the morning the Nauru files were published, Pezzullo immediately asked senior staff to undertake an urgent review of the files. The files show a limited number of other documents on which Pezzullo appears to have handwritten.

When Save the Children issued a release setting out that it gave the department all of its incident reports in October 2015, Pezzullo handwrote: “We need to explain our knowledge of the full suite of SCA files — esp. handover of data in Oct 2015.”

Save the Children had delivered all of its incident files directly to the immigration department in October 2015. But the department appears to have taken few steps to integrate these files into its systems and examine whether they had been handled appropriately.

The correspondence from the department shows that the taskforce had gone to extraordinary lengths to create new IT infrastructure to analyse the materials. The 2,116 leaked reports set out the assaults, sexual abuse, self-harm attempts, child abuse and living conditions endured by asylum seekers held by the Australian government. A Senate inquiry was convened following the publication of the files.

The immigration department has been contacted for comment.