Gordon Campbell has published a column that falls into a trap to which many others have succumbed during the referendum campaign: assuming he remembers what the Royal Commission on the Electoral System said, without actually checking. His column argues:
The voting referendum is really a contest between the only systems that the public know first hand: MMP, and the old FPP system. In a pure head to head contest, FPP would fail. To give it a helping hand, National candidates have been either explicitly endorsing or dog whistling for SM as a moderate, middle ground alternative. It is no such thing, of course. SM contains only a pathetically token trace of proportionality and entrenches big party power. (The Royal Commission dismissed it with disdain as barely any improvement at all on FPP.)
(Emphasis mine)
The Royal Commission did reject Supplementary Member, and they did consider that MMP was superior, but to declare that it "dismissed it with disdain", or that it view SM "as barely any improvement at all on FPP" is a massive overstatement (as is the claim, formerly made elsewhere, that "SM was roundly rejected by the Royal Commission").
Chapter 2 of the Report of the Royal Commission on the Electoral System was devoted to considering which voting system we used, and it considered (mostly briefly) a range of voting systems: First Past the Post, At-large voting, Points Voting, Limited Vote, Single Non-Transferable Vote, Cumulative Vote, Second Ballot, Alternative Vote, Approval Voting, First Past the Post with supplementary seats, Supplementary Member, Party List, Single Transferable Vote, and Mixed Member Proportional.
It rejected most of these quickly, but its assessment of the Criteria for Judging Voting Systems it formulated determined that three alternatives to first past the post were worthy of in-depth consideration: MMP, STV and SM. And about supplementary member, it noted in introducing it that:
2.81 There is, however, a more promising scheme which we have called the Supplementary Member (SM) system…
2.83 SM is a serious and considered attempt to improve our present system…
And concluded:
2.114 The Commission recognises that SM has considerable appeal. It improves on the plurality system in a number of ways. First, it would give representation to significant minor parties. Second, because almost all the list votes would count towards the election of candidates, electors in safe seats would have a more effective rol than under the present system. Third, it would enable the parties, particular major ones, to protect a limited number of particularly able members in marginal seats. Fourth, it would provide a way of increasing the number of MPs but avoid the disruption to constituency boundaries that would be caused by a significant number of single-member constituencies. Fifth, it would, because of the list, be likely to enhance the representation of Maori voters as well as voters belonging to other special interest or minority groups. Sixth, it would lessen somewhat the disproportionality between the major parties.
Even in weighing up SM's negatives, it was reluctant to fully rule it out:
2.115 Nevertheless, the Commission is of the view that SM does not go far enough in meeting the fundamental objection to the plurality system in respect of the relationship between seats and votes. Those objections would still be powerful under SM, even though the minor parties might be somewhat better off. We are reluctant to rule out SM altogether, however, until we have seen whether MMP or STV can overcome the objection to both plurality and to SM without introducing too many disadvantages of their own.
In contrasting SM with the proportional STV and MMP systems the Royal Commission concludes:
2.179 … With regard to SM, we are conscious that a complete move away from plurality represents a major change and that there might be attractions in making lesser modifications to our system aimed at remedying some of its defects in a more gradual and incremental manner. However, we do not consider SM sufficiently overcomes the key deficiencies of plurality. In terms of fair representation of the supporters of political parties and other groups and interests, it is a palliative rather than a true prescription for improvement.
This is clearly a rejection of supplementary member, but it is no disdainful dismissal. Rather, SM was viewed by the Royal Commission as a serious and considered attempt to improve first past the post which held considerable appeal, but which did not go far enough in meeting the fundamental objections to first past the post in respect of the relationship between seats and votes. Not quite as easy to put on a billboard, but if you're going to invoke the Royal Commission as supporting your argument, worth checking and getting right.