The second referendum on the New Zealand flag begins today, with more than three million voting packs being delivered to mailboxes over the next week.

Robert Peden, chief electoral officer, said voting packs would be arriving in people's mailboxes in the next few days.

"They'll arrive between today and Friday 11 March. Your voting pack will include your voting paper, a return freepost envelope and full information on how to complete your paper. It will also include information about the two flag options voters will be choosing between."

The referendum runs from today until March 24.

Voters will be asked to choose between the current New Zealand flag and the preferred alternative design selected in the first referendum in late 2015. The flag that receives the most votes in the second referendum will be the official flag of New Zealand.

"On your voting paper, you will be asked to tick the flag option that you want to be New Zealand's flag," Mr Peden said. "Make your choice, then get your vote back in the post by 21 March."

Voters should get their voting paper by Friday 11 March. Enrolled voters who do not receive their pack by then should visit or freephone 0800 36 76 56 to get a replacement paper. They can also request a replacement paper if they make a mistake, lose, or destroy their paper.

Once completed, voting papers should be placed in the return envelope, and posted in a New Zealand Post postbox by March 21 to ensure they get back in time.