All Black great Dan Carter and other high-profile Kiwis are trying to boost support for changing the flag with a new video.
In the second and final referendum next month, voters will choose between the current flag and the Blue and Black Silver Fern flag designed by Kyle Lockwood.
The video launched today by the Change the New Zealand Flag campaign, features a line-up of eminent citizens from the worlds of business, culture, politics and sport.
However, recent polls suggest most people want to keep the old flag.
Campaign chairman Lewis Holden insisted "momentum for change" was building.
A recent poll of 1000 people by Curia Research showed the campaign was gaining ground, with support for the old flag dropping to just 56 percent from a high of 69 percent last September.
"We've always said we're the underdog here," Mr Holden said.
"We're working as hard as we can to communicate with as many New Zealanders as possible in the lead up to this referendum, which is now about a month away.
"So there's still time to carry that conversation on."
One group opposed to change is the Returned and Services Assocation.
Its head David Moger said the charity did not have the cash to spend on glossy marketing, unlike the campaign for change, whose supporters had very deep pockets.
"Their supporters have very deep pockets and they're prepared to put their hands deep into them," he said.
"That's completely their choice of course, where they spend their money.
"If there were any individuals wanted to do that for us, we'd be very, very appreciative."
The RSA membership and many other New Zealanders were proud of the current flag because it represented their history, he said.
"There are some people that say we have to get rid of the Union Jack because it's a colonial thing.
"But the reality is we have a history that comes from there, but now we are a nation that's mature enough to recognise our history, both the good parts and the bad parts.
"A lot of great things have happened under that flag, and in the future, a lot of great things will happen under that flag."