"To get it over the line you can't just rely solely on National voters because there is always going to be a contingent of National voters that just don't support change. That's quite legitimate - this isn't about a political issue, it's about whether you support change or not."
The latest One News Colmar Brunton poll showed a quarter of voters planned to vote for change and about three-quarters of Labour supporters planned to vote for the current flag.
Mr Key said that would be an "unlikely result" if people were voting purely on the issue at hand, given those on the left had traditionally supported change and Labour had made calls for change in the past.
"It isn't logical 75 per cent of Labour voters would be wanting to retain the existing flag. There is no question one of the factors slowing the process down is the rally cry from the leadership of Labour and the Greens to vote against it, simply to give the Government a black eye."
Mr Key said he was surprised by the number of younger voters polled who planned to vote for the current flag.
Labour leader Andrew Little said he suspected Mr Key's silence was an attempt to distance himself from his "pet project" because of the polls.
Mr Little said it was possible turnout would be low if the polls meant people assumed it was a fait accompli that the current flag would stay.
"That can lull those people opposed to a flag change into a false sense of security. So it may be a combination of him wanting to distance himself from it so it does not look like his project any more as well as if he says nothing maybe the turnout will be low and there might be a majority on the low turnout that support the change in flag."
Mr Key did not believe the issue had affected his own credibility, saying his personal polling has held up throughout.
Mr Little has criticised the referendum process and the design of the alternative flag, chosen in a referendum last November.
Mr Key said New Zealand was the first country to put the issue through a referendum process.
The view on the street
On Thursday, voting papers for the flag referendum go out to New Zealanders. The Herald went to Flagstaff Tce, in Devonport, for the first in a series of what Kiwis on the street think.
Hayley Stevenson
Which flag are you voting for?
"The old New Zealand flag. Absolutely."
Why?
"Because of the critical causes in New Zealand that $26 million ... could have been spent on. Stuff the identity factor, I don't think that is something that is relevant."
What do you think of the referendum generally?
"I think it was very biased. I think having [Red Peak] in was a bit of a whitewash to confuse people and drug up the masses. The new one looks like a tea towel. I mean they couldn't even get the colour of it right when they put it on the bridge, could they?"
Inge Rudolph and Ron Ackroyd
Ron: "We voted for the similar one that came first in the referendum but I object to the black background. I think it's black because of the All Blacks which is stupid.
"I think it should have been blue. We get too carried away with All Blacks, black everything. One other thing that concerns me is that recently I read that while we had 25 per cent of kids into poverty, that's now up to 30 per cent and we've just spent $25 million on a flag referendum and that's not necessary. We could have stayed with the flag and spent that money on kids, food in schools, whatever."
Inge: "There was one with the red background with the fern and I liked that one. I object to the amount of money that's been spent on this and I'm wondering what the real issue is, if it's because they want to have a different profile for New Zealand because the Australian and the New Zealand flag are quite close together and being mistaken for the other."
Jay Strydom
Which flag are you voting for?
"I'm voting the new flag."
Why?
"Just to differentiate from the old flag and I like the fern and the colours are good. I actually like the Red Peak one the most but obviously my one didn't win, but it's fine."
Do you think it's worthwhile having the referendum?
"Yes, I think so."
