UPDATED 3.45pm The Prime Minister and Deputy Prime Minister are urging Kiwis to have their say in the flag referendum.

Postal voting opens tomorrow in the ballot that'll see the country either stick with its current flag, or adopt a new silver fern southern cross design.

Prime Minister John Key said it's a rare opportunity for Kiwis to have a say on which flag they believe best represents this country.

"It will probably never happen again in the lifetime of most New Zealanders that will vote in this referendum, and I think they should choose the flag that represents them best."

Deputy Prime Minister Bill English wants everyone to have an input into the decision saying all eligible voters should have their say.

Green Party co-leader James Shaw has given his take, saying he doesn't favour the Lockwood design.

"I don't think it efficiently tells the story of who we are as a nation, and so I'll be voting to keep the existing flag on this occasion."

And Labour MP David Parker said he will also stick with the status quo.

"I'm voting for the existing flag, not the bath towel," he said.

To enrol on line go to www.elections.org.nz or visit a PostShop and will out an enrolment form.