Senator Sinodinos wrote to the committee conducting the inquiry last Wednesday confirming his refusal to appear on Thursday, despite the Senate directing him to do so to answer questions. He argued it has never been Senate practice to order ministers to appear before an inquiry.

His letter disingenuously raises the precedent of former Labor Senator Mark Arbib, who also refused to appear before an inquiry in 2010 into the Rudd government's Pink Batts program. Senator Sinodinos argued he should be treated the same way. However, he’s stretching the truth, for in that case witnesses were not directed to appear by the Senate, but rather requested to appear by the committee conducting the inquiry, which could not compel their attendance.

The inquiry was prompted by a NSW Electoral Commission statement last month that the Liberals used the Free Enterprise Foundation to "channel and disguise" donations, including from banned donors, before the 2011 NSW election.

The commission said it relied on evidence in 2014 to the NSW corruption watchdog by party officials and "through them" evidence of the "involvement" of finance committee members including Senator Sinodinos "in the arrangements touching the foundation".

Senator Sinodinos chaired the NSW Liberal finance committee and was party treasurer at the time but has insisted he had no knowledge of the foundation being used to channel illegal donations.

In an unusual move, the Senate passed terms of reference specifically directing Senator Sinodinos "to appear before the committee to answer questions".

Greens Senator Lee Rhiannon, an inquiry member, said Senator Sinodinos' refusal to appear had "increased the web of intrigue around his political donation activities".

"With the NSW Liberals already under a cloud over past political donation scams, it is not a good look that possibly the closest confidant of the Prime Minister refuses a Senate directive to appear and give evidence," she said.

So what the committee now needs to explore is whether Senator Sinodinos is in contempt of the Senate, considering there was a clear direction passed by a majority of Senators. The NSW Electoral Commission has withheld $4.4 million in public funding from the NSW Liberals until the party formally discloses who donated $693,000 to it via the Free Enterprise Foundation before the 2011 election.

Given Senator Sinodinos has ignored a clear direction to appear, the committee should now explore whether he is in contempt of the Senate. Senator Sinodinos’ failure to appear is also testament to the Liberal party’s “deny and defy” approach to scrutinising political donations.

Senator Sinodinos is becoming a problem for Prime Minister Malcolm Turnbull. This was an opportunity for Senator Sinodinos to put his case and clear the air on the various allegations made about his activities as NSW Liberal Treasurer. Mr Turnbull’s failure to pull him into line brings to mind former Prime Minister Abbott’s failure to censure then Speaker Bronwyn Bishop over the “Choppergate” affair.

Silence on this issue is not a wise course of action, especially going into an election.