The legal advice, which was tabled in Parliament on Tuesday afternoon by the Speaker of the House of Representatives, Tony Smith, intensifies an incendiary debate over border security just as the government risks losing a vote on the bill.
Opposition Leader Bill Shorten has made a series of concessions to improve the chances of gaining support from the Greens and the crossbench for amendments that could be passed by the lower house on Tuesday afternoon.
The vote is on a bill that was amended in the Senate by Labor, the Greens and crossbenchers in early December, triggering the question over whether the upper house has made an appropriation and flouted the constitution.
Attorney-General Christian Porter wrote to Mr Smith on Sunday to tell him of the legal questions and to ask him to choose the “appropriate course of action” in line with previous decisions where the House of Representatives has rejected amendments from the Senate.
Mr Porter also provided legal advice from the Solicitor-General, Stephen Donaghue, but told Mr Smith he would “appreciate” the Speaker not circulating the advice.
Mr Smith took a different view, however, and chose to table the letter and the advice in Parliament, making it public to all.
“I have advised the Attorney General that, as Speaker, it is important that I ensure in this instance all material available to me is also available to all members of the House,” Mr Smith said.
The Attorney-General says in his letter that this is no “mere technicality” because the Senate cannot appropriate public funds.
“Were the House to ignore the requirements of Sections 53 and 56 of the constitution, the Senate would be given the power to dictate the expenditure of public revenue, with potentially crippling effects on public finances,” Mr Porter wrote.
“This would be all the more perverse given that the government of the day, whatever its political complexion, rarely enjoys majority support in the Senate.”
The Solicitor-General advice was dated February 7 and Mr Porter's letter to the Speaker is dated February 10.