The Information Commissioner is investigating the Department of Home Affairs over its failure to process freedom of information requests within the statutory time limits.
Government departments have 30 days to process freedom of information requests, although can extend that time frame for various reasons.
In a statement on Friday, the commissioner said more than half (56 per cent) of requests for non-personal information made to the department in the past financial year had not been processed within that time limit.
Home Affairs secretary Mike Pezzullo had been grilled about his department's performance in turning around freedom of information requests in Senate estimates on Monday.
Deputy secretary Cheryl-anne Moy said the department was on track to receive about 21,500 requests for information this year.
But crossbench senator Rex Patrick noted it only had about 74 staff working in the freedom of information division.
"I'll tell you the thing that disturbs me most in these numbers: the number of deemed refusals, which simply means you ran out of time. You never bothered to seek an extension; you just let it run over time," Senator Patrick said.
"My personal experience in the FOIs that I've done to Home Affairs - and I do FOIs across a number of departments - is that you put something in and, after the 30-day statutory period, nothing has been received. So you acknowledge the initial FOI and then that's it - nothing occurs.
"That seems to me to be somewhat disrespectful, in the context that you should at least seek an extension and keep people informed as to the fact that you're not going to hit the time frame."
But Mr Pezzullo said the department complied with more than 80 per cent of freedom of information requests.
Mr Pezzullo said he was not looking to shift resources into the area "because they would have to come from frontline operations".
"I've got an obligation to address finite resources across all of my risk areas. I would have to take resources away from frontline operations, whether it's counter-terrorism or counter-foreign interference," he said.
Mr Pezzullo also told senators the rate of decisions from his department being overturned by the Information Commissioner was low, despite the "heightened tempo and increased volumes" freedom of information officers faced.
And while he urged his chief operating officer "to take out the red tape" as much as possible, Mr Pezzullo said he was yet to be convinced that the team was not performing well.
However Information Commissioner Angelene Falk said she had launched the investigation after a number of complaints and review applications about Home Affairs.
Ms Falk told estimates on Tuesday "timeliness had been an issue" for Home Affairs for several years.
"In 2016-17, only 25 per cent of requests were processed within the statutory time frame. That has been significantly improved over the last two years to 74 per cent processed within time," Ms Falk said.
"One of the key factors in relation to that, as I reported last financial year, was that the Department of Home Affairs instituted an administrative access program. So it shows you the value of those proactive administrative access programs, but more work needs to be done in relation to timeliness with that particular department."
A department spokesman said it received the highest volume of FOI requests of all Commonwealth agencies.
"The department is continuing to improve its compliance with the statutory time frames contained in the FOI Act," he said.
"For the 2018-19 financial year, the department finalised 75 per cent of requests within statutory time frames. The compliance rate for the 2019-20 financial year to date is 82.5 per cent, with a total of 5,533 cases finalised."
However the compliance rate fell to 62.5 per cent for the 444 non-personal requests processed so far this financial year.
The announcement comes after The Canberra Times revealed more than 60 per cent of decisions made by the Information Commissioner last year overturned decisions made by government departments, with hundreds more settled without a formal decision being made.
The Information Commissioner received more 928 applications for reviews of Freedom of Information requests in 2018-19, an 80 per cent jump on the 510 applications received four years ago.