Parties target particular seats for different reasons, but always in their own interests.
Some seats in this year's election are potentially make-or-break for smaller parties. That's where tactical voting comes in. Audrey Young looks at four electorates that could shape the next government.
All votes are considered equal when it comes to the Party vote because the votes for each political party are tallied on a national basis, no matter where you live, and those votes determine the number of MPs each party gets.
But the same cannot be said of the second vote, the Electorate vote, which elects local MPs.
Some electorates turn out to be more equal than others.
They acquire importance because they can affect the survival of smaller parties, and can sometimes affect the potential shape or influence of the government.
Such electorates are strategically important and the best known current example is Epsom, but several others in this election, to varying degrees, could be strategically important – Northland for New Zealand First, Te Tai Hauāuru for the Māori Party and Auckland Central for the Greens.
Voters in strategically important seats sometimes vote tactically - not necessarily for the party or party's candidate they support but to help the party they support.
Having a hold on a seat also gives supporters of smaller parties the comfort of knowing their vote won't be wasted, given that a party must win an electorate seat to have any presence in Parliament if it polls under 5 per cent nationally.
Jim Anderton's Alliance, and then Progressive voters had that comfort after the Alliance collapsed because his hold on Wigram was never in doubt and similarly Peter Dunne's United Future had the anchor of his hold on the Ōhariu seat.
Winston Peters' New Zealand First supporters had that comfort in the first MMP election in 1996 because his hold on Tauranga was not in doubt – although he lost it in 2005.
A strategic seat does not necessarily need a formal deal between parties or even a symbolic "cup of tea" between leaders as sometimes occurs, although it can help.
A credible poll in an electorate can also affect voter behaviour, by presenting a disincentive to vote for a "losing" candidate or creating momentum for someone leading or close to leading.
Northland is potentially a strategic electorate for New Zealand First because it has been so in the past and, on paper at least, could be again.
But so much has changed since the start of this year when it was considered a real prospect.
Until Covid-19 struck, it looked as though it would be a close-run election, giving Labour an incentive to help NZ first, its smaller Coalition partner, to get back to Parliament by securing an electorate seat.
First-term National MP Matt King holds it with a small majority of 1389 after beating NZ First leader Winston Peters in 2017. On paper it would It would take just 16 per cent of the Labour candidate's vote to go to NZ First candidate for NZ first to regain it.
But Labour no longer has an incentive to help – even with an informal nod and wink to its supporters.
It is polling so well itself it might not need any partners, and New Zealand First has not given a commitment it would again support a Labour-led government.
Besides which, NZ First candidate Shane Jones does not have the same appeal as Winston Peters and has been a difficult minister for Prime Minister Jacinda Ardern.
Peters took Northland from National in the 2015 byelection because Labour voters got behind him with the blessing of then-Labour leader Andrew Little.
It reduced the National Government's majority by one. But that small shift meant it was no longer possible for just National and Act to pass legislation between themselves. It did not affect a lot of laws but it stymied, for example, planned reform of the Resource Management Act.
Jones is standing in Northland for the third time but for the first time for New Zealand First. He stood for Labour in 2005 and 2008 against National's John Carter.
His allocation of hundreds of millions of dollars to Northland through the Provincial Growth Fund this term appears not to be paying off in support.
A 1 News Colmar Brunton poll in early August suggests that Jones is well behind on 15 per cent, with King in the lead on 46 per cent and Labour's Willow-Jean Prime on 31 per cent.
If Jones had been within striking distance of King, Northland's Labour voters may have instinctively shifted support to Jones without encouragement from the leadership.
But with the gap so wide, it would take explicit approval from Jacinda Ardern for that to happen, and that does not appear likely.
Act's hold on the National-supporting Epsom electorate dates back to 2005, with three Act MPs having held the seat since then: Rodney Hide, John Banks and current leader David Seymour.
It looks as though that grip is about to pay dividends again for the party as polling suggests it will boost its MPs beyond the solitary MP it has returned for the past three elections.
The Epsom anchor means Act supporters across the country can be almost certain their vote won't be wasted if the party polls under 5 per cent.
The same happened in 2008 when Act received only 3.65 per cent of the Party vote across the country but it got five MPs because it had won Epsom, and it became a support partner to the National-led government.
Split voting is a common tactic in strategic seats, although not all split voting is done for tactical reasons. But it is very common in Epsom.
Just over 60 per cent of Epsom electors who gave their party vote to National last election gave their candidate vote to Act's David Seymour. Act got only 696 Party votes in Epsom compared to National's 22,875.
Taking tactical voting to extremes, some Labour supporters voted for National's candidate, finance spokesman Paul Goldsmith, in an unsuccessful bid to keep out Seymour and Act.
Goldsmith will continue to campaign for the Party vote. National will need to get about 34 per cent Party vote nationwide for him to get back in, assuming they keep the same number of electorates and the same amount of Party vote is wasted as last election.
Tactical voting has had a mixed reputation under MMP, sometimes described as dirty deals or coat-tailing, but it has been around from the beginning.
Prime Minister Jim Bolger in 1996 encouraged National voters to support Act's Richard Prebble in Wellington Central, although Act eventually got over 5 per cent.
The Green Party has targeted Auckland Central as an electorate seat to win but the conditions, on paper at least, don't favour success.
The Greens came a poor third last election behind the Labour candidate, Helen White, who came within 1581 votes of reclaiming the seat from National's Nikki Kaye.
White is back again this time and now that the popular Kaye has retired, Labour supporters may well be motivated to stick with the candidate who didn't miss by much last time.
Prime Minister Jacinda Ardern is expected to campaign with White this afternoon.
The high-profile Green candidate MP Chloe Swarbrick is better known than Roche and can be expected to poll higher. Likewise National's Emma Mellow is not expected to do as well as Nikki Kaye.
But a win by Swarbrick would still require a massive shift on the centre-left.
Based on the 2017 result, the Green candidate would need to persuade about 97 per cent of the Labour candidate supporters to back her in order to beat the National candidate.
The decision by the Greens to target Auckland Central for an electorate vote may cost it party votes.
Auckland Central has been a good harvesting ground for Party votes for the Greens – the fourth-best-performing electorate last time.
Swarbrick is running a two-ticks campaign – Green for the Electorate and Party vote.
But for those who like to split their vote, and two-thirds of Green voters did in 2017 across the country, she runs the risk of getting an electorate vote but not the all-important Party vote.
The alternative theory is that standing the high-profile Swarbrick in the high-profile seat of Auckland in itself helps the Green Party profile and party vote across the country.
And it is possible that if the party looks in danger of not making it back over 5 per cent, Labour's Auckland Central voters would instinctively help their preferred support partner back – with or without Ardern's approval.
The first and only time the Greens have won an electorate seat was in 1999, when they had split from the Alliance, and it was not certain they would make the 5 per cent threshold (they scraped in with 5.16 per cent).
Labour leader Helen Clark gave her blessing for Labour voters to support the Greens co-leader Jeanette Fitzsimons in Coromandel which she held for one term only.
Of the seven Māori seats the western seat of Te Tai Hauāuru is regarded as the least safe, for two reasons.
First it has the smallest majority of any of the seven seats, all of which are held by Labour.
And second, arguably, it has the strongest Māori Party challenger in co-leader Debbie Ngarewa-Packer up against Labour's weakest candidate in Adrian Rurawhe.
The strategic importance of her winning the seat is not that it would necessarily bring in any other MPs but it would give the party a presence and a profile in the Parliament after having been out for three years.
That could give the party a platform to rebuild support for a comeback in 2023. If she did win the seat and the party vote allowed a second MP to come in from the list, it would be Rawiri Waititi, the Waiariki candidate and No 2 on the list.
Rurawhe, a second term MP and Assistant Speaker, holds the seat with a 1039 majority.
Affiliating to Ngati Apa, he is well known in the Whanganui part of the electorate, and he has deep connections to the Ratana Church, founded by his great grandfather Tahupotiki Wiremu Ratana.
Ngarewa-Packer is well known in the Taranaki end, having grown up in Patea and now lives in Hawera where she is chief executive of Ngāti Ruanui.
She has the backing of former Māori Party founder Dame Tariana Turia, who held the seat for 12 years.
The advantage Rurawhe has is that he is part of a larger party organisation which can be harnessed for on the ground work and is an MP in a party with a hugely popular Prime Minister.
Māori Party co-leader, former Labour MP and former mayoral candidate John Tamihere, is standing in Tāmaki Makaurau and while he has a high profile, he is considered to have less of a chance than Ngarewa-Packer.
Peeni Henare's majority is larger and he is more visible in his new portfolio of Civil Defence, as well as Whānau Ora and Associate Health.
But perhaps the biggest advantage of the Maori Party this election is that all seven of Labour's Māori electorate MPs would be dead certain to get back into Parliament on the list if they did not win their seats.
That wasn't the case last election when none of them went on the party list.
The Māori Party could argue that with tactical voting, voters could get both Labour and Māori Party candidates, and increase the level of Māori representation in Parliament.
NZTA says it is liaising with police, warns Aucklanders to be prepared for disruptions.