Vote 2017: Herald Election Forecast has it very tight in race to form next government

Both Labour and National are projected to win around 51 seats in the poll on September 23.

What a difference a week - and a poll - makes.

Last week's One News Colmar Brunton poll saw Labour widen its lead over National to 4 percentage points.

Last night's Newshub Reid Research poll put National almost 10 percentage points ahead - 47.3 per cent to 37.8 per cent.

Where does the truth lie?

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 that 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.

After last week's Colmar Brunton poll, our model predicted 54 seats in Parliament for Labour, 48 for National. That was a change from the previous update, released earlier last week, that forecast the big two parties would be deadlocked at 51 seats each.

Now we've fed the results from the poll released last night into our model. It gives National the edge - but only just.

It is projected 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.

At this stage neither National or Labour has a clear path to 61 without New Zealand First support or a comeback from the Greens.

For the first time the model shows Greens falling well short of the 5 per cent threshold parties must reach to win seats through MMP if they don't win an electorate.

New Zealand First is projected to win 11 seats - ensuring it remains a consistent factor in an uncertain race.

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.

A new update includes different methodology for Epsom, which is unlike most other electorates due to tactical voting.

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

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