New Zealanders are being put under undue influence as they prepare to vote in the final flag referendum later this week, the Labour Party claims.

Postal voting opens later this week. Voters will be asked to choose between the current New Zealand flag and and the Kyle Lockwood-designed 'Silver Fern' flag.

The Flag Consideration Panel maintains it's been politically neutral, but Labour Leader Andrew Little believes the process was geared to meet the needs of the Prime Minister, which many say is designed to be a 'legacy project'.

"John Key made it clear that he wanted a flag that had a silver fern on it. Lo and behold, the vast majority of the forty flags that went up for consideration by the panel and cabinet had silver ferns on it."

Little is voting to keep the current flag, and he fears that if Kiwis vote to change the flag to a design that is clearly unpopular then there might not be another chance to change it again for another hundred years.

"We can get a better flag. It might be a few years before we have this debate again but I would rather would rather hold on to what we've got knowing that when the circumstances arise and the right conditions exist then we can get a much better flag."

However Flag Consideration Panel Chair Professor John Burrows said that while there's been a political debate, it hasn't affected the panel's work - no matter how challenging that work was.

"We've tried to keep as remote from it as we possibly can. We have maintained our neutrality," Burrows said.

"I don't think it's influenced us at all."

"In other countries where there has been talk about flag change, it's always been contentious. It's never been straightforward process, so we knew it was never going to be easy."

"I think at times we underestimated how difficult it was going to be."

A number of high-profile New Zealanders - including former Prime Ministers Jim Bolger and Jenny Shipley, former Air New Zealand boss Rob Fyfe, and rugby player Dan Carter - are backing a new flag. They appeared in a slick video production organised by former National Party candidate Lewis Holden.

Richie McCaw, who was recently declared 'New Zealander of the Year', has also urged a change in national symbol.

Early last week, the alternative Lockwood flag was pulled down from an official building in Te Puke and replaced with a pair of underwear and some shoes.