Ms Mills voluntarily appeared to answer questions at a senate hearing on Wednesday.

Her sacking last month – the reasons for which were not yet known – meant she was no longer obliged to be grilled at parliamentary inquisitions.

Ms Mills correctly pointed out she was one of three experienced, respected secretaries who had left in the space of eight years, rightly inferring those with only a superficial understanding of the department's history could blame her for all its shortcomings.

Her predecessor, Alan Thompson, retired early in January 2012 amid a Senate committee probe into allegations of DPS maladministration and bullying in the department.

Another DPS secretary, Hilary Penfold, resigned in 2008 to take up an appointment to the ACT Supreme Court bench.

When the department, which has 800 staff and a $120 million a year budget and has been the subject of continual reviews, makes a mistake it does so in front of an audience of vocal critics – politicians, their staffers and journalists.

A trivial issue can quickly become national news, as seen during the 2007 "stroganoff affair" when Labor MP John Murphy complained about the size of the beef stroganoff portion served to his wife and alleged rudeness of a cafeteria staff member.

But recent questioning of Ms Mills over two incidents during robust parliamentary hearings has been justifiable.

Senators have been correct in exposing the dangerous lack of transparency at DPS when it awarded a photographic contract to artist Anne Zahalka.

They understandably questioned links between Ms Mills and her Sydney neighbour Zahalka. The intensity of inquiries increased because there was little paperwork showing how the decision was made.

While she told subordinates she would remain "at arm's length" from the process, the lack of a paper trail regarding the process demonstrates a major failing at a department under her watch.

At the end of 2014, two-and-a-half years after Ms Mills took on the job, the Australian National Audit Office confirmed poor contract management at DPS was a systemic problem. Few contracts audited had strong value for money arguments.

The second serious issue Ms Mills faced related to her evidence about allegations CCTV footage was used to spy on a whistleblower's communication with Senator John Faulkner.

Senators implied Ms Mills should resign when the Senate privileges committee said she had provided misleading evidence – a finding she disputes – about when she was told about the CCTV footage.

In talking personal attacks, Ms Mills might point to a letter sent by Senate Clerk Rosemary Laing to British officials saying it would be "embarrassing" if Ms Mills won the job of UK House of Commons clerk. While it appears the letter and ensuing controversy cost her the job, this is not where the majority of pressure on her has come from.

Correction: The original version of this story was altered to make it clear that Hilary Penfold resigned as secretary of DPS in 2008 to take up an appointment to the ACT Supreme Court bench.