Voters will be asked to choose between the preferred alternative flag design and the current New Zealand flag.
The flag that receives the most votes will be the official flag of New Zealand.
The preferred alternative design was selected in late 2015 in the first referendum.
"On your voting paper, you will be asked to tick the flag option that you want to be New Zealand's flag. Make your choice, then get your vote back in the post by March 21."
If a paper is lost, destroyed or a mistake is made, voters can also request a replacement paper.
To ensure they are received in time, completed voting papers should be placed in the return envelope and posted by Monday, March 21.
The deadline for enrolment was on Wednesday, and as under the legislation only people enrolled by the start of the voting period were able to participate.
Meanwhile, a Rotorua Lakes Council spokeswoman said they did not intend to fly the alternative flag next to the current flag.
Instead, it will continue to fly the New Zealand flag and the Rotorua Lakes Council flag at the council building.
-More information about voting in the referendum is available at www.elections.org.nz or by calling 0800 36 76 56.