Vote 2017: Herald Election Forecast predicts Labour victory - and outcome of every electorate battle in New Zealand

Eight days out - and the latest Herald Election Forecast has Labour edging National with 55 seats in the next parliament to 51.

Our prediction model was updated after the release last night of the latest 1News Colmar Brunton poll, which had Labour and National 4 percentage points apart at 44 and 40 respectively.

Both were up one point, and New Zealand First was down three points to 6 per cent. The Greens were up 2 points to 7 per cent.

The Herald's Election Forecast crunches data from every major poll conducted in the past year and the results of every election since 1999. Check out the latest update in full, and find out how it was built here.

We can't guarantee it's going to predict exactly what will happen on September 23, but statisticians believe that collecting more data, over a longer period, is more likely to create a true picture of where New Zealand's voting intentions lie.

The data is fed into the model, which simulates thousands of outcomes. Those possible outcomes create a band of probability for each party.

The upper and lower estimates for projected seats mean Labour and National could be deadlocked on the same number of seats.

The update, created after the release of last night's poll, forecasts six seats for the Greens, although the lower estimate remains zero seats.

The most probable path to forming a government remains, for both major parties, an alliance with New Zealand First, who are projected to win nine seats.

Before last night, he previous update to our model was based on the Newshub Reid Research poll release last Thursday. That had National almost 10 percentage points ahead - 47.3 per cent to 37.8 per cent.

The result was that our model projected National to win 54 seats, Labour 53.

To form a government, a party or coalition needs at least 61 out of 120 seats, or to reach a confidence-and-supply agreement.

The forecast model also includes predictions for candidates by every electorate. These are based on previous election results and take into account any polling that has taken place for a particular electorate.

There is different methodology for Epsom, which is unlike most other electorates because of tactical voting.

The visualisation also includes candidate and party vote predictions for Maori electorates. However, this is perhaps the hardest part of forecasting because of tactical voting and lack of consistent polling at electorate level.

We'll continue to update the Herald Forecast as new polls are released. It's becoming more responsive to the polls as we get nearer to the election on September 23.