Who's going to win the Northcote byelection? There are no independent polls, no big indicators, just lots of little clues. They're not easy to read.

Two weeks ago, National said its campaign polling revealed it had an 8 per cent lead, while Labour claimed, from its own polling, that National's lead had fallen from 6.4 per cent to 2.1 per cent.

If National was right, its candidate Dan Bidois would probably win with a 1500 majority. If Labour was right, National might squeak home with fewer than 400 votes, or be swamped altogether by a rising tide of support for Labour and its candidate, Shanan Halbert. Here are the numbers.

In the 2017 general election National's party vote was 18,005 against Labour's 12,639. That's a majority of 5366. To put that another way, National had 48.7 per cent of the total party vote and Labour had 34.2 per cent. That's a lead of 14.5 per cent.

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The polling by both parties suggests this race is quite a bit closer than that.

Are there lessons to learn from other byelections? We've had 10 this century, of which three were in Maori seats, one did not have a National candidate (Mt Albert, 2017) and one was dominated by the personality of Winston Peters (Northland, 2015): it's hard to generalise from any of them.

Which leaves five: Mt Roskill (2016), Christchurch East (2013), Botany (2011), Mana (2010) and Mt Albert (2008). Four were Labour seats that remained Labour, while Botany was a National seat that stayed with National.

There's the first lesson: byelections don't unseat incumbent parties. Northland was the only occasion that has happened in the last 25 years. And you have to go back to 1930 to find a byelection where the Government took a seat off the Opposition.

Next lesson: byelections have lower turnouts. In Mt Roskill, Christchurch East, Botany and Mt Albert (2008) the average turnout was only 51 per cent of the turnout in the closest general election.

Candidate Dan Bidois. Photo / Doug Sherring
Candidate Dan Bidois. Photo / Doug Sherring

Mana had a higher turnout (67 per cent), but that result was also an outlier, because National put up high-profile Cabinet minister Hekia Parata, who squeezed the Labour majority. Neither candidate in Northcote this time is an MP, let alone a Cabinet minister.

If 51 per cent of Northcote voters from 2017 turn out for the byelection, there will be 18,867 votes cast. With that number, if the 2017 relativities are repeated, National's Bidois will win with 9182 votes against Labour's Halbert with 6445. That's a lead of 2737 votes.

But if National's claim of an 8 per cent gap carries into the final figures, National will win by 1509 votes. And if Labour's claim of a 2.1 per cent gap showed up in the final figures, National's victory margin will be just 396 votes. Assuming a 51 per cent turnout.

How important is the trend? National's lead in Northcote in the 2014 election was 29 per cent. By 2017 it was down to 14.5 per cent. If that's a trend that continues, as suggested by National and Labour's poll claims from two weeks ago (an 8 per cent or 2.1 per cent lead for National), it points to a Labour victory.

Against that, external nationwide polling this year suggests there has been almost no movement in support for the major parties since the general election. Then, National gained 44.4 per cent of the vote and Labour 36.9 per cent. That's a lead of 7.5 per cent.

If the gap is 7.5 per cent in Northcote on Saturday, assuming a 51 per cent turnout, National will win by about 1400 votes.

But, what about the minor parties? The Greens are standing in this byelection and so is Act, but NZ First has stayed out of it. Parties with little chance of winning usually do very badly in byelections, because they are first-past-the-post contests. Voters tend to treat them as a battle between the two leading candidates.

If that happens in Northcote, Labour will pick up Green votes. The Greens won 6.7 per cent in the electorate in 2017 and are polling around 6 per cent nationwide now. Those votes could swell the Labour number and damage National's chances. Act won 0.7 per cent.

Meanwhile, advance voting is well ahead of the 2017 figures. Voting opened two weeks before polling day and in the first four days was 77 per cent higher than in the first four days of the general election. That jump continued on Saturday, with 1625 people voting on that day alone, up from 1116 on the equivalent Saturday last year.

Since then the increase has slowed: after the first nine days it was 38 per cent higher than the first nine days in 2017. On Tuesday this week there were actually 65 fewer votes cast than on the equivalent Tuesday last year.

Still, 38 per cent is big. What's going on? Labour supporters say it's a surge for Labour, on the theory that bigger turnouts favour their side. But at least two other explanations are possible.

One is that there are more advance polling booths this year, including booths in shopping malls. The rise in early voting may simply be a result of greater convenience.

Another explanation is that this is the continuation of a trend towards early voting. In the 2014 general election 26 per cent of all votes were cast early. In 2017 that rose to nearly 40 per cent. The trend to date suggests this could be the first election in which more than half the votes are cast ahead of time.

Or will the total turnout be unusually heavy? Or maybe the heavy advance voting of the first week will dry up in the second? We'll know on Saturday night.

Next question: Is Northcote a National seat or a "bellwether seat" where voters like to support the Government of the day? The electorate has been on the winning side in every election since 1972, except 1984, 2005 and 2017. Arguably, even 2005 and 2017 fit the pattern because Northcote voters might have thought National was going to win those elections.

Which leads to a final question: if byelections don't unseat incumbents, who is the incumbent here? National holds the seat but Labour leads the Government. While the two tyro candidates go hard at each other, will Northcote be voting for them, or will it treat the byelection as a referendum on the Government – and on Prime Minister Jacinda Ardern?

It seems clear from the numbers that despite National's current majority, Labour could win this election. So is Northcote going to jump on the Ardern train, or is it fed up already?