Here’s why (Commonwealth Of Australia Constitution Act 1.3.40):
Questions arising in the House of Representatives shall be determined by a majority of votes other than that of the Speaker. The Speaker shall not vote unless the numbers are equal, and then he shall have a casting vote.
The only situation in which it is permissible for the speaker’s casting vote to have an effect is that where an even number of members’ votes are split exactly in half. For the sake of argument, assume a situation in which one member is absent and where the speaker supports the motion in question. The vote (i.e. excluding the speaker) is 74-for/74-against, so the speaker gets a casting vote and the motion passes. Oakeshott’s proposal would require one of the opponents of the motion abstain, leaving a result of 74-for/73-against, causing the motion to pass without the speaker having to cast his vote. So far, the arrangement achieves the same outcome as the arrangement required by the constitution.
To understand the problem with Oakeshott’s proposal, consider the situation where – for the same motion – everyone except the speaker votes and the result is 74-for/75-against. In this case, the constitution is clear, the speaker shall not vote and the motion will fail. Oakeshott’s proposal would again require one of the opponents of the motion abstain, leaving a result of 74-for/74-against, causing the numbers to be equal and the speaker’s casting vote to be considered, in this case causing the motion to pass, a result contrary to that required by the constitution.
The language is simple, the contradiction is obvious. Any act of Parliament which passed because of Oakeshott’s “extra” vote would risk immediate collapse on first contact with the High Court, which would rightly expose the party whose member’s abstention enabled this to work to a voter backlash.
I am astonished that Oakeshott’s lawyers didn’t point this out to him as soon as he suggested it, or worse, that he suggested a legal arrangement without consulting a competent lawyer (echoes of One Nation’s disastrous tax policy).