The one-seat rule is one of the matters up for consideration in the review of MMP and it is ripe for change.
Under current rules, a party that wins a seat is able to get extra seats in parliament regardless of how many part votes it wins.
This gives a lot of power to voters in a single electorate which is one of the criticisms levelled at First Past the Post.
There might be a case for allowing extra MPs if the party reaches a lower threshold than the 5% required by those which win no seats, but I would like a higher threshold than a single seat by itself.
If there is no change to the requirement for parties to have more than 500 members the need for winning more than just one electorate seat will be even greater.