OPINION: It's been a campaign of twists and turns ... but could there still be one more twist?

Bill English wrapped up a final frantic race up country for votes with a speech to a packed pub on Auckland's Viaduct with the promise "we are going to win".

But on the latest polls, it's still too close to call.

STUFF Bill English wrapped up a final frantic race up country for votes in Auckland.

It has been assumed NZ First will hold the balance of power if neither National or Labour and the Greens have the numbers to govern. But senior Nats have told Stuff - don't rule out an approach to the Greens instead.

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The scenario under which that might happen would be if Labour and the Greens clearly couldn't get over the line, and the Greens looked like a more attractive partner than NZ First.

KEVIN STENT/STUFF Greens co-leader James Shaw said he was confident he could trust Labour would argue a fair deal on their behalf.

Metiria Turei's departure from the Greens co-leadership seems to be what lies behind National's belief that a deal may be possible - she was always cast as an implacable opponent to any deal with National. James Shaw is seen as being more of a pragmatist.

But National would only be prepared to make environmental concessions - the Greens' social and economic policy platform would be seen as a step too far. Big concessions on climate change policy would also be a stumbling block.

On both those counts the Greens would likely rule themselves out of a deal - co-leader James Shaw has made it clear economic and social policy have the same priority as environmental policy.

There is a view within National, however, that a deal with the Greens would be more forward and future looking than any deal with NZ First.

One concern is what is seen as an erratic list of NZ First bottom lines, but there is also an acknowledgement that National was exposed on environmental issues like dirty water in the campaign.

That's why National insiders say an approach to the Greens should not be ruled out.

But the tight race means there are also a range of potential outcomes from a relatively small number of votes.

That factor, and the number of special votes cast, means it may not be immediately obvious on the night who will form the next Government.

But there is also another twist - the flood of early votes means a result might be clear within a few hours of the polls closing.

As many as 40 per cent of votes may have been cast already, and most of those votes would have been counted before the polls close at 7pm Saturday.

Counting will begin from 9am.

If those 40 per cent of votes show a clear gap, that would almost be a big enough sample to call the race.

But it's possible no-one will be brave enough to do that - especially with NZ First and the Greens just hanging on above the 5 per cent threshold on current polling.

In the case of the Greens, meanwhile, a large number of special votes could be decisive as they have traditionally picked up support when the final vote is released.

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