Key Points:

The Maori Party is one of three parties that could contribute to an increase in the size of Parliament from 120 MPs after the election, on the basis of today's Herald-DigiPoll survey.

Assuming the four Maori Party MPs keep their seats, and party leaders with seats keep theirs, the Maori Party, United Future and the Progressives would contribute to an increase in the size of Parliament.

Each of the three parties would add one extra seat to make a Parliament of 123 seats on the basis of today's poll.

Any seats over the standard 120 in Parliament are called the "overhang".

The overhang is a contentious issue because the greater the overhang, the higher the majority needed for a party to form a Government.

And in theory, a party that wins more than 50 per cent of the vote at the election might not get more than 50 per cent of the total seats in Parliament.

United Future leader Peter Dunne has been especially vocal about what he sees as the potential dangers to democracy in the overhang, and says it could pervert the will of the voters.

The Maori Party has the highest likelihood of contributing to a big overhang - if it wins a large number or all of the seven Maori electorate seats with the electorate vote but wins a small party vote. The bigger the electorate haul, the bigger the overhang.

Some Labour and Green supporters want to encourage tactical voting by Maori Party followers - party-vote for Labour and the Greens and thus starve the Maori Party of party votes - to deliberately increase the overhang and make it harder for National to get a majority.

But the party says it is open to both and is campaigning for the party vote as well as the electorate vote.

The overhang is created when a party wins more electorate seats than the party vote gives them.

The party vote determines what proportion of seats each party gets when the 120 seats are divided among qualifying parties. If a political party wins more electorate seats than the party vote delivers it, they increase the size of Parliament.

The current Parliament has 121 MPs - the one extra MP being earned by the Maori Party last election when it won 2.1 of the seats. That translated into three seats but the party won four.

The Maori Party in today's DigiPoll survey got 2.4 per cent of the vote, or three seats.

Assuming its four MPs keep their seats, the Parliament would expand to 123 seats and the majority required would be 62 votes in Parliament (instead of 61 for 120 seats).

National however could govern alone with 63 seats.

Assuming the Maori Party's vote is 2.4 per cent and it wins six of the seven electorate seats, the overhang would run to 125 seats (three Maori, one United Future, one Progressives), and the majority required would be 63 votes in Parliament - National's number of seats in the poll.

And assuming the Maori Party won all seven Maori seats, the overhang would stretch to 126 (four Maori party, one United Future, one Progressives) and the majority required would be 64 votes - National's 63 plus one from either United Future or Act.