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NZ Post says about 30 boxes have been removed since 2016, although it could not provide the actual locations.
A spokesman said the person responsible for maintaining the database was on leave.
NZ Post says a national analysis showed there was no correlation between the reduction in the number of post boxes in communities and lower voter turnout.
"In fact, there was often a higher voter turnout in more recent years (since there has been a reduction in the number of post boxes).''
As well as post boxes, voters could cast votes at postal agencies.
Anyone who does not know where their nearest post box is can enter their address on the NZ Post website's post box locator tool.
Dunedin voters planning to post their ballot papers are being encouraged to do so a week before polling day to ensure they arrive on time.
Although voting closes at noon on October 12, deputy electoral officer in Dunedin Sharon Bodeker said if people were posting their papers , "we're encouraging them to post these by Saturday, October 5''.
However, votes can also be hand-delivered to the civic centre in the Octagon until noon on October 12.
There will also be ballot boxes at the temporary special voting booth, which will be at various locations around the wider city during the voting period, Ms Bodeker said.
More details about this would be available later in the month.
"Our key message is encouraging people to vote early, however they choose to do this.''
Voting papers will be sent out from tomorrow, September 20 to September 25.