Prince Charles and his wife Camilla the Duchess of Cornwall arrive in Wellington on Wednesday for a week-long royal visit.
But the Green Party says it is surprised by the timing of the tour, given that later this month voting begins in the New Zealand flag referendum.
In less than three weeks voting papers will be sent out to people, asking them to rank five alternative flag designs.
The Greens say the visit could backfire on the Prime Minister, who argues the current flag with its Union Jack is outdated.
Green co-leader James Shaw said he was surprised the royals were touring so close to the referendum, given John Key's campaign to change the flag.
"I was quite surprised at the timing because John Key has a history of maximising royal visits for photo opportunities and so on," Mr Shaw said.
"And so it seemed an odd choice to schedule a royal visit at exactly the same time as he gets his pet project flag referendum through. Because obviously you would assume that a royal visit would lead to a resurgence of Union Jack loyalty."
The Minister in charge of the Flag Referendum, Bill English, declined to be interviewed about the timing of the royal visit.
But the Labour Party's spokesperson for internal affairs, Trevor Mallard, did not think it would have much of an influence.
"The current flag is not one of the options in the November referendum, so I can't really see it being a big issue. I'm relatively confident the royals won't express an opinion on whether they like Red Peak or not," Mr Mallard said.
Mr Mallard said the flag referendum was unpopular with the public anyway, and predicted nothing will change.
"We think it's a waste of money, we think it will fail."
ACT MP David Seymour said that while some believed the royals and the Union Jack were a package deal, Canada was proof that they were not.
"If you look at the history of our country we have progressively become more and more independent of Great Britain, they're no longer our biggest trading partner, not even close.
"And yet we still have deep cultural, constitutional, and linguistic ties and I don't think you have to have the Union Jack on your flag to get a royal visit - the first time I ever saw a member of the royal family was Princess Anne in Western Canada," Mr Seymour said.
The Electoral Commission will begin posting the first flag referendum voting papers on 20 November and said everyone would have them by the 27th.
Voters have to have returned their voting papers by 11 December.
In March next year the most popular alternative will run off against the current flag in the second referendum.