Australia’s environment department is unlawfully withholding more than 10,000 pages of freedom of information documents from the public, including internal records on Adani and the Angus Taylor grasslands affair.

The department has failed to place documents on its FOI disclosure log for the past 10 months, meaning material it has released to individual applicants is not visible to the wider public.

The failings, first reported by the Mandarin, are a breach of FOI law, which compels government agencies to publish documents online within 10 working days of giving them to the initial applicant.

It means more than 10,000 pages have not been published on the log, including internal records on its decision to approve the controversial groundwater plan for the Adani coalmine and on Taylor’s interactions with an investigation into land-clearing by a company he and his family part-owned.

Guardian Australia understands the department’s conduct has been the subject of an official complaint to the information watchdog, the office of the Australian information commissioner (OAIC). The OAIC typically does not comment on ongoing investigations.

The department has blamed an increased workload for failing to update the disclosure log since 17 December last year.

But FOI statistics show it generally has a smaller workload than most other agencies, which have kept their logs up to date much more efficiently.

The last available statistics show the environment department is not even in the top 20 agencies for FOI requests. That list is topped by the home affairs and human services departments, which have managed to keep their disclosure logs relatively up to date.

The environment department received 123 FOI requests in 2017-18, compared with 14,177 for home affairs. Human services received 6,238 requests and its log is completely up to date.

A department spokesperson said there had been a “significant upsurge” in FOI requests and internal review requests, creating a backlog of work for its staff.

“The department has uploaded on to its disclosure log documents released under FOI decisions made during 2018,” the spokeswoman said. “However, the department has not been in a position to update its disclosure log for decisions made after December 2018.

“This is as a result of a significant upsurge in the number of primary and internal review FOI requests lodged with the department, which has required the department to prioritise its FOI processing resources towards the critical task of responding to primary FOI requests.”

Experts have criticised the department’s failure to meet its obligations.

Peter Timmins, a lawyer and highly-regarded FOI expert, said it showed a broader problem with the federal government’s attitude toward FOI.

“It shows a broader problem really about the state of FOI if we have agencies that can disregard quite clear obligations to make documents released publicly available on their disclosure log,” Timmins said. “The broader problem is that I think without appropriate leadership – and that really is at the highest level of government – about the importance of transparency and integrity, we see these breaches occur.”

Timmins said he had some sympathy with departments, who were expected to deal with FOI requests with ever-dwindling resources.

“I’m sure there are quite a few agencies that are struggling to keep up with their obligations under the act, I have some sympathy for the department of environment on that front,” he said. “However, the law is the law.”

In July, an anonymous individual used FOI to apply for all the documents the environment department had failed to upload to its disclosure log.

The request was refused in August. The department said at the time that it expected to make the documents available on its disclosure log “shortly”.

“The department is currently experiencing a backlog in uploading documents on to the department’s disclosure,” the department’s acting general counsel, Kate Lalor, said at the time.

“The department has taken steps to reduce the backlog and will continue to do so. It is anticipated that documents will continue to be uploaded on to the disclosure log in tranches over approximately the next month.”

Two months on the documents still have not appeared.