It's the last day to vote in the referendum, and Prime Minister John Key is optimistic the polls are wrong and this evening New Zealand will be flying a new flag.

But that's not because he knows something we don't -- speaking to More FM this morning, Mr Key said he hasn't been given any heads-up on the result.

"I wish I did. I mean, you think you'd get all this power when you're Prime Minister, and you find out the same time as everybody else."

Voting ends at 7pm and a preliminary result should be known by 8:30pm.

"I'll find out probably 15 minutes before everyone else, but not any more than that," said Mr Key.

His optimism comes not from polling by news outlets -- which have consistently shown the existing flag comfortably beating Kyle Lockwood's design -- but internal National Party polling and his own interactions with the public.

"I can look at the polls and see that a lot of people don't want to change, equally on our polling each and every week in the last month it's gone up a lot, in terms of people wanting to change," he told More FM.

"I might be proved wrong, but the reason why I think there's at least a chance is that a lot of people have come up to me… and said, 'I wasn't voting for change. I wasn't going to vote for change. I picked up the pen and paper, looked at the options and voted for the new one.'"

More people have voted in this second part of the referendum than the first -- at least 1.9 million -- and Mr Key expects it to top 2 million, making it one of the most successful referendums in New Zealand history.

"When some people said this isn't an issue that either captivates the minds of New Zealanders or they're not interested, then actually I think the numbers have proven that to be incorrect," he said yesterday.

"People have been very engaged and they've gone out and voted in extremely high numbers."

Whichever way the vote goes, Mr Key hopes Kiwis get behind their flag.

"If they retain old one, as much as I'd like to change to the new one, I hope people fly the old one and wear it and are proud of it."

The official results will be revealed on March 30. If you haven't yet voted, you can still drop off your papers at any PostShop.

Newshub.