Section 53 of the constitution states the Senate “may not amend any proposed law so as to increase any proposed charge or burden on the people” – a provision that gives the government grounds to challenge the crossbench bill.
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.
Constitutional expert Anne Twomey, professor of law at the University of Sydney, said the Solicitor-General's opinion was "pretty fair and accurate" but that the final decision was up to the Parliament itself, with completely different views on Section 53 between the two chambers.
Section 53 is "non-justiciable" and therefore a court will not decide if a law is valid.
"So it is really a matter for the House of Reps," Professor Twomey told Sky News.
Raising the stakes further, Professor Twomey said that if the government was now asserting that the bill was a money bill, the government would be exposed to claims it should lose power if it lost a vote on the bill.
"If the government is now asserting that this is a money bill, then, if the bill actually gets passed, against the wishes of the government, that would be an indication that the government has lost control over the finances of the country," she said.
"That's criticial in terms of confidence and loss of government."
Adding to these comments, Professor Twomey said the medical transfer "raises the stakes considerably" over the government's fate.
"Once a government loses control of government finances, it is obliged to resign or seek a dissolution," she told The Sydney Morning Herald and The Age.
"See, for example, the fall of the Fadden Government in 1941 when the budget was amended to reduce it by the nominal sum of 1 pound. So there are risks involved."
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.
In his advice, Dr Donaghue acknowledges the bill may not “clearly, necessarily and directly” result in additional expenses because the cost of the additional panel members might be covered by the reallocation of funds from elsewhere.
Dr Donaghue rejects this because the amendments themselves would be sufficient to constitution a charge or burden on the people and, in any case, the government might not cut funding elsewhere to cover the new costs.
“Indeed, I have been instructed that the government has no intention of proceeding in that way,” he wrote, in advice dated February 7, last Thursday.
A crucial element of his advice is that the bill in its current form, as amended by the Senate, “could not have been introduced in the Senate” because that would breach Section 53 of the Constitution.
From that, it follows that the bill would require a message from the Governor-General under Section 56 of the Constitution, which provides that a bill that appropriates money must not be passed unless the purpose of the appropriation “has in the same session been recommended by message of the Governor-General to the House in which the proposal originated”.
“In the past, the House has responded to a problem of this kind by disagreeing to purported Senate amendments and then, after obtaining and a Governor-General’s message recommending appropriation, proceeding to make amendments in the same terms,” Dr Donaghue wrote.
“It is a matter for the House as to whether it wishes to take that course.”
Dr Donaghue emphasises that it is “ultimately for the House of Representatives to decide” whether it wishes to proceed or not.