New Zealand's 113-year-old flag will win next year's binding referendum, according to an online poll, but the March vote could be close.

Fifty-six per cent of respondents to an online Herald survey yesterday said they would vote to retain our current design, which officially became our flag in 1902.

Forty per cent of the 7500 who took part in the unscientific poll said they would back Kyle Lockwood's black, white and blue silver fern, the provisional winner of stage one of the referendum.

In the preferential vote, the flag pulled 50.53 per of eligible votes, just ahead of his red, white and blue fern which drew 49.47 per cent.

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In the referendum, 9.7 per cent of votes were declared informal and were not counted.

In the online survey, 3 per cent said they wouldn't vote and a further 1 per cent said they would spoil their vote.

Prime Minister John Key yesterday urged New Zealanders to take next year's second referendum seriously.

"Even if you don't like me, if you want a new flag I encourage you to vote for it," he said.

And he also defended the $26 million cost.

"It comes down to nationhood and the promotion of New Zealand. It's worth it," he said.

"The economics of changing a flag overwhelmingly support doing that, because you have much greater recognition of New Zealand.

"As a country, I think we'd use it more and would rally behind it," he said.

The final result of stage one of the first referendum will be announced on Tuesday.