Every politician has influence outside their constituency........ it's called Parliament.
I believe what @Rikkitic is getting at is that his influence is disproportionate. If the locals really want Peter Dunne personally then it wouldn't matter if he joined the National Party. If he joined the National Party, he'd be a back bencher. But because he has his own political party, National give him the proverbial baubles in order to secure his vote.
Therefore, he is only in Parliament because of very local popularity (if it were not so, his party vote would be bigger, yes?) but his influence far exceeds the influence of any other local MP. Although it is not unfair in the sense that no rules have been broken, it certainly breaks the spirit of MMP in the sense that the opinions of those living in one single electorate disproportionately overrule every other, thanks merely to a technicality.
Thanking it does not make it right
Again Every politician has influence outside their constituency........ it's called Parliament.