Leader Andrew Little says the vote on whether or not New Zealand should change its flag should be ditched before any more taxpayer money is wasted.
"Millions of dollars could be saved if the prime minister called a halt to this hugely expensive, and highly unpopular, vanity project," Mr Little said.
New Zealanders don't care about changing the flag and now is the wrong time to do it, he said.
"Extraordinary sums are being spent on it at a time when the country is facing economic headwinds and households are struggling."
The two-step postal referendum is expected to cost around $26 million.
Prime Minister John Key faced questions in parliament on Tuesday from Greens co-leader James Shaw about the level of interest in the flag debate.
Mr Key said more than 10,000 alternative flag designs had been submitted, showing people were keen on it.
Mr Shaw asked Mr Key whether he accepted that only 25 percent of New Zealanders wanted to change the flag.
"I think we are ultimately going to have to wait until that final referendum in early 2016 to see whether New Zealanders want to change the flag or not," Mr Key said.
"I believe passionately that a new flag can be used as part of a way of showing how proud we are of our country and how we unify a multicultural society."
The Flag Consideration Panel is now reviewing the designs to come up with a long-list of between 50 and 75 alternative flags.