The alternative flag will fly on the Auckland Harbour Bridge ahead of a referendum to decide if it will replace the current New Zealand flag -- a move slammed as desperate by NZ First leader Winston Peters.
The black, white and blue silver fern design will fly on the bridge from Friday, and will fly alongside the current flag.
"Having the two flags flying side by side around New Zealand will help people compare the designs before making a decision in the final flag referendum, which runs from March 3 to March 24," Deputy Prime Minister Bill English said.
The decision has been criticised by Mr Peters, whose party is staunchly opposed to the flag change process.
"They have no statutory right to do it -- this is just a blatant attempt by the Prime Minister to put his fern flag on the same level as our national flag," Mr Peters said.