MMP aims to allocate seats in parliament in proportional to voter support for political parties. Two-choice voting can remove most of the distortions that arise from the use of party-vote thresholds. But should an indpendent win an electorate seat, or a party win more electorate seats than its party votes justify, then an <overhang> is created. This, in the NZ system, is addressed by creating extra seats. The Maori Party has won overhang seats a few times.
But this system seems to invite manipulation, by electing independents who are not really so. A party would encourage candidates in safe seats to become independent <friends of the party>. The home party would not stand a candidate against them. These <friends> would be expected to win their electorate seats, and create an overhang.
A similar idea has already been used in Epsom, where ACT has been able to win the electorate seat, while National benefitted from the party vote. In this case, I think the move helped ACT pass the one-seat threshold for being allocated seats.
This idea might be acceptable in isolated cases, but imagine if major parties has a dozen of these arrangements each. The result could be 20-seat overhangs.
One might hope that the voters would reject such moves as unfair, and punish those practising it. Or it could become an accepted political manoeuvre.
I think that the current electoral system review should ensure that the rules make such manoeuvres more difficult. But this does not seem easily achieved.
One method would be to require, instead of votes for person and party separately, a single vote for a person and their party. Thus a vote for a Blue party candidate would also be a party vote for the Blue Party. Two-choice party voting would work well with such a system, except that the first choice would be for person and first choice party, the second for a second choice party.
See here for a Stuff article that mentions this issue.
A two-choice MMP voting paper might look like this: