One person who received the letter said it was "highly unusual" that the request was coming directly from the senator's office and not the Department of Finance.

"Why were we being slapped with this the day before Christmas? It was highly unusual and the threatening subtext of the letter was basically demanding loyalty," the person told BuzzFeed News.

Several people received the letter many months after leaving Fierravanti-Wells's office. According to Australian government protocol, staffers are to sign confidentiality agreements before they start working in an MP's office and "on separation" from employment. BuzzFeed News asked several former ministerial chiefs of staff, who said they'd never heard of agreements being sent out months after employees had left.

According to one recipient, it was the unusual timing of the letter that had several people asking questions.

"It was to reiterate that we weren't to talk about the manoeuvring in the office because she was going into pre-selection," the recipient said.