The total cost of the country's flag referendum has been released, with figures showing the project came in under budget.

The provisional cost has been revealed as $21.8m, almost $4m less than what was initially budgeted.

That's despite the late addition of the red peak option, which cost just under $270,000.

The most expensive aspect of the process was the first postal referendum, coming in at a cost of $9.

After two referendums, 57% of New Zealanders voted to retain the current flag with the remainder voting for the Kyle Lockwood alternative.