Nauru is blocking the release of information requested under Australian freedom of information laws relating to refugees and asylum seekers held on the island, according to court documents.

Documents filed by the immigration department to the Administrative Appeal Tribunal cite Nauru’s objection as a key reason to block a freedom of information request.

The department had put in an application for extra time to appeal a decision of the information commissioner to release the documents.

The tribunal approved the extension last week, allowing the department to appeal despite missing thedeadline by five months. The lengthy delay was blamed on a lapse by an “unidentified officer”.

Immigration department blames Nauru freedom of information delay on 'junior officer' Read more

It comes four weeks after Guardian Australia obtained an email from the department that suggested FoI requests relating to Nauru had been “frozen”.

The information sought under the FoI, filed in July 2015 by the Melbourne-based lawyer Alex Cuthbertson, was the number of people of a certain religion, ethnicity and language group on Nauru.

The request was for raw numbers only. The department opposed it under the personal privacy provisions of the Freedom of Information Act, arguing that the population of asylum seekers on Nauru was so small that the information could be used to identify individuals.

In his decision overturning that objection and approving the release of information in March, the information commissioner, Timothy Pilgrim, said that the likelihood of being able to identify anyone based on the information sought was “so impractical that there is almost no likelihood of it occurring”.

The deadline for appealing that decision passed in April but the department did not file an appeal with the tribunal until September, after receiving the letter from Nauru and after Cuthbertson sent a follow-up letter in August demanding the release of the documents.

In an affidavit on the court file, the assistant director of the department’s FoI division, Kelly Tulloch, said that the file had been inadvertently closed by a junior officer.

Tulloch said when she received Cuthbertson’s email in August she realised the request concerned Nauru and had a conversation with colleagues about the “high degree of sensitivity with the disclosure of any information from, or concerning, the processing of protection claims by the government of Nauru”.

The department then filed an appeal under both the personal privacy provisions of the Freedom of Information Act, claiming Pilgrim had erred in his decision, and under international relations provisions.

Immigration official says department is 'freezing' release of documents about Nauru Read more

Cuthbertson told a tribunal hearing earlier this month that she required the information as background for the high court case of an asylum seeker who had been held in Nauru and was now held in Australia. She said she was concerned, based on Guardian Australia’s reports, that the information was being blocked just because it concerned Nauru.

Cuthbertson’s client is fighting to stay in Australia. She has been guaranteed 72 hours’ notice of any plans to return her to Nauru and Cuthbertson said her case had been prejudiced by the failure to release the information.

In her reasons for approving the application for appeal the tribunal’s deputy president, Stephanie Forgie, said the department’s explanation for the delay was “understandable” and that she did not believe there had been any “improper motive”.

Forgie said she did not believe the department email published in Guardian Australia indicated the department was seeking to delay access to information, because the email was written by an FoI officer “and not by an officer with line responsibility in a policy or operational area in the department”.

She continued: “The email does not lead me to the conclusion that the department’s FoI section was the instigator of that delay. It does not lead me to take the view that the premature closure of the file was anything other than an unfortunate error.”

Forgie also did not believe Cuthbertson had been prejudiced by the delay, saying that: “In so far as the resources of the tribunal enable expedition, the matter can be expedited.”