Australia’s immigration minister was extensively briefed about the impact prolonged detention was having on children held on Nauru, official correspondence shows, before he dismissed reports of sexual assault and abuse as “hype” and “false allegations”.

Peter Dutton has repeatedly sought to diminish the significance of the Nauru files, published by the Guardian – more than 2,000 leaked incident reports that contain accounts of self-harm, abuse, sexual assault and other incidents from the offshore centre up until October 2015.

The Guardian can reveal that the offices of senior members of the Australian government – including prime minister, Malcolm Turnbull, attorney general George Brandis and Dutton – all received an extensive dossier in May 2016 that outlined the ongoing harm to children held on Nauru and the “numerous child rights violations” that had occurred.

Dutton also personally received an earlier briefing in August 2015, to which he personally responded, and met with chief executive of Save the Children Australia, Paul Ronalds, after receiving the first written report.

The child rights agency – which provided welfare services on Nauru from 2013 to 2015 – compiled data collected internally by their staff, including from incident reports, case files and other testimony in order to provide a snapshot “that goes beyond anecdotal reports”. The correspondence indicates that the briefing showed that the rate of child exposure to reportable incidents indicated a deterioration of their mental wellbeing, and the escalating risks of sexualised behaviour, depression and self-harm as the length of detention increased.

The full report remains secret. Save the Children Australia is unable to release it due to laws that criminalise the disclosure of information about Australia’s immigration detention system passed by the government in 2015.

In response to questions from the Guardian about the extent of their briefings to the government, Save the Children Australia’s director of policy and public affairs, Matt Tinkler, said: “The prime minister and immigration minister should not be surprised about the misery, abuse and harm being suffered by children on Nauru, which was again evidenced in the documents published by the Guardian Australia last week.

“Over the last year Save the Children provided senior members of the government with two detailed reviews of the incidents our staff witnessed, heard and reported while working in the Nauru centre.

“These reviews presented a full picture of the damage effective indefinite detention is having on asylum seeker and refugee children and their families. The analysis was based on 1,000 incident reports and 1,800 monthly caseworker reports from the Nauru detention centre.”

Dutton said last week that while he would not tolerate “any sexual abuse whatsoever” there were “some people do have a motivation to make a false complaint”.

Official correspondence obtained by the Guardian – and published in full here – between Save the Children and senior members of the Coalition government show that Dutton was warned as recently as May 2016, just as the federal election was under way, that offshore detention on Nauru was continuing to harm children.

The May 2016 letter, which was addressed to Brandis, Turnbull and Dutton from Ronalds, said: “Using the evidence gathered through data analysis, staff observations and other credible sources, the enclosed report on ‘Child Asylum Seekers in Nauru’ identifies significant and numerous child rights violations in this context.

“Children were, among other things, detained for extraordinary periods of time, disproportionately negatively affected by the experience of detention, frequently exposed to harm, violence and abused, and were often separated from their parents.”

It recommended that the child asylum seekers and their families not be transferred to or forced to remain in locations where they were at serious risk of harm, that they be subject to ongoing independent oversight and that they be given access to effective remedies to address human rights abuses.

A senior adviser to the department of the prime minister acknowledged receipt of the May 2016 briefing, and said it had been referred to Dutton “who has portfolio responsibility for this issue”. The Guardian understands the prime minister did not personally receive the briefing.

Dutton declined to respond personally, and instead a departmental official wrote in May 2016 that they noted the recommendations made and said Australia “remains committed to the regional processing and settlement arrangements in place on Nauru”.

But the May 2016 briefing was not the first that Dutton was provided with this level of analysis. He was also given an earlier briefing based on Save the Children’s analysis of their incident reports and casework reviews in August 2015.

Ronalds wrote in a letter to the minister: “The data review is a rigorous analysis of all of the incident reports and monthly casework reviews which SCA has on record in relation to the 281 children who have been held on Nauru. It provides an evidence-based case study to inform discussions about the impact that detention has on the welfare of children.”

Dutton responded personally, and said: “With respect to the review of data held by SCA in relation to the welfare of children at the Nauru RPC, I reiterate that I take matters of child protection very seriously and the Department of Immigration and Border Protection shares this attitude.

“The implementations of recommendations arising from the Moss review has resulted in improvements to centre security and care for children and vulnerable persons in the Nauru RPC [regional processing centre].

“The department is working closely with the government of Nauru (Gon) to assist with the development and implementation of a child protection framework on Nauru.”

On the last official count at the end of June, 442 people – 338 men, 55 women and 49 children – were held in the Nauru regional processing centre. The other offshore centre, on Manus Island in Papua New Guinea, was holding 854 people, all men. On Wednesday Dutton announced the Manus Island detention centre would be closed but gave not details about the future of detainees.

Australia’s policy has been criticised regularly by the UN.

The release of the correspondence shows the extensive behind the scenes lobbying efforts by Save the Children Australia that have been under way over a lengthy period of time.

The government is facing a likely Senate inquiry into the response to allegations of abuse and sexual assault. The backing of the inquiry by Labor, the Greens and key crossbench senators indicate the inquiry is likely to be successfully established.

The prime minister, attorney general and Dutton were all contacted for comment regarding the Save the Children reports.