This was more votes than in any previous postal referendum, but as a proportion of total voters it was lower than a referendum on compulsory pensions in 1997.
Just a few hours remain in the second referendum, and the Electoral Commission plans to announce a preliminary result this evening.
Ballot papers are still pouring in. Another 170,000 votes were received by the Electoral Commission on Thursday, raising the total vote to nearly 2.1 million people.
This was more votes than in any previous postal referendum, but as a proportion of total voters it was lower than a referendum on compulsory pensions in 1997.
The high levels of engagement were proof that the flag debate was worth having, Prime Minister John Key said today - regardless of the outcome.
"When some people said this is an issue that either captivates the minds of New Zealanders or they are not interested, then actually the numbers have proven that to be incorrect," he said.
New Zealand First leader Winston Peters had criticised the referendum, partly because of its $26 million cost. Mr Key said the New Zealand First-sponsored referendum on superannuation cost the same amount and was resoundingly defeated.
Polls are consistently showing that New Zealand will keep its Union Jack and Southern Cross design, which has been the national flag since 1902.
However, a global poll of more than 10,000 people shows support for a new flag.
A Buzzfeed post asked readers: "Which flag should New Zealand choose?"
Kyle Lockwood's flag received 9095 votes - 76 per cent of the vote - while the current flag received 2794 votes, 24 per cent of the vote.
Mr Key made a final pitch for his preferred design, a blue and black flag featuring a silver fern which was selected out of 10,300 public entries.
"It's fundamentally about taking the Union Jack off and putting the silver fern on," he said.
"To me, that's a symbol that we're known by. That's a symbol that we put on our Commonwealth war graves. That's a symbol that's internationally recognisable."
The Prime Minister kicked off the flag referendum process in March 2014. The process has been divisive and MPs from the major political parties are generally voting along party lines - Labour against change and National in favour.
Mr Key said the referendum would not leave a mark on his leadership.
"I never actually saw it as part of my legacy, one way or the other," he told reporters. If the flag changed, future generations would not recall the leader of the time, he said.
"I can't tell you who the Canadian PM was when they changed the flag. Most Canadians probably can't. I've got lots of things I would like to mark down as my legacy ... but the flag isn't going to be one of them."
Labour leader Andrew Little, who is voting against change, has said that the debate could be held again when New Zealand considered becoming a republic.
Voters have until the end of today to make up their mind. Ballot papers which are postmarked before 7pm, will still count in the referendum.
A final result will be confirmed next Wednesday once all late votes have been counted.
When is the flag vote announced?
A preliminary result will be confirmed around 8pm tonight. A final result will be declared on Wednesday the following week.
What happens next?
If the alternative flag wins, the design will gradually be rolled out across the country, probably making a debut at the Olympic Games in August.
If the status quo wins, the issue is expected to be retired for some time, at least until New Zealand debates a change to a republic.
Can I still vote?
Yes, but your ballot paper must be in a letterbox before the end of today. It will only count if it is postmarked before 7pm.
Postal referenda
2016: Second flag referendum
Votes: 2,097,124 (as of Thursday)
Turnout: 66.1 per cent
2015: First flag referendum
Votes: 1,546,734
Turnout: 48.78 per cent
2013: State-owned asset sales (citizens-initiated)
Votes: 1,368,925
Turnout: 45.07 per cent
2009: Right to smack children (citizens-initiated)
Votes: 1,684,402
Turnout: 56.09 per cent
1997: Compulsory superannuation
Votes: 1,988,650
Turnout: 80.3 per cent
