Opposition legal affairs spokesman George Brandis has told the coalition a deal it signed early this month altering the role of the speaker is a "plain violation" of the constitution.
In an attempt to woo key independents, both Labor and the coalition agreed that the speaker in the next parliament would be automatically paired for votes.
But now Senator Brandis, a senior counsel, says such an arrangement would "circumvent the plain meaning of section 40 of the constitution".
"For that reason alone it should not be countenanced," Senator Brandis writes in his legal advice to Opposition Leader Tony Abbott.
"In my opinion, pairing arrangements cannot be extended to the speaker."
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
Pairing occurs when members with opposing views agree not to vote - effectively cancelling each other out.
Section 40 of the constitution states that the speaker "shall not vote unless the numbers are equal and then he shall have a casting vote".
Senator Brandis says the section makes clear "that the speaker may not vote on the question before the chair, unless the numbers are equal, in which event he has a casting vote".
"To extend pairing arrangements to the speaker would, in effect, be to treat the speaker's casting vote, proleptically, as if it were a deliberative vote, which is a plain violation of the prohibition in section 40," the opposition's legal affairs spokesman writes.
Senator Brandis argues that if the framers of the constitution intended otherwise they would have made it clear - as is the case for voting in the Senate.
Section 23 states that the president of the upper house is entitled to a deliberative (or ordinary) vote but not a casting vote.
When the result is tied, the question is negatived.
But Senator Brandis says he believes it's arguable the speaker cannot be paired because the constitution actually "imposes an obligation upon the speaker to resolve the tie" in the House of Representatives.
Legislation passed under a pairing arrangement could subsequently be ruled invalid by the courts, he argues.
However, the Queensland senator fails to discuss section 50 of the constitution in his written advice.
It states that either house "may make rules and orders with respect to ... the mode in which its powers, privileges, and immunities may be exercised and upheld (and) ... the order and conduct of its business and proceedings either separately or jointly with the other house."
The government has asked the solicitor-general to advise whether the reform deal struck by Labor, the coalition and the independent MPs on September 6 is valid.
The coalition says if the advice shows pairing is constitutional it will honour the original deal.
Prime Minister Julia Gillard doesn't want to lose a vote by nominating a speaker without pairing in place because the numbers are so tight - 76 to 74 in the lower house.
