Australia's tactics used to turn back asylum-seeker boats must be kept secret or they will damage international relationships, Home Affairs Minister Peter Dutton was warned.
A departmental briefing, obtained under Freedom of Information laws and published by BuzzFeed on Monday, reveals what Mr Dutton was told when he took over the Home Affairs portfolio in December last year.
The document said revealing information about Operation Sovereign Borders "may affect Australia's relations with foreign governments" and put international agreements at risk.
"Information that may reveal the location, capacity, patrol and tactical routines relevant to Navy and (Australian Border Force) vessels and air assets is not released," the department told Mr Dutton.
"Such information can undermine the government's tactical advantage over people smugglers who seek to use this information to avoid or trigger detection or to precipitate a search and rescue response.
"Additionally, the release of this type of information may enable an exploitation of sensitive tactics, techniques and procedures employed.
"This information may also affect Australia's relations with foreign governments and undermine the potential for international agreements and co-operation necessary between operational agencies."
The document also reveals the backlog of processing the asylum-seeker claims from the 50,000 boat arrivals during Labor's term in office.
About 14,300 asylum-seekers had their claims processed by December 2017, but another 14,800 were still waiting.
"We estimate completion of the initial assessment of the caseload by December 2021," the department said.
The report also shows Australia is kicking significantly more people out due to character visa cancellations.
There were 76 people whose visas were cancelled on character grounds in 2013/14, but three years later in 2016/17 that figure rose to 1284.