Thirty-eight people have been referred to the police for allegedly voting twice during last year's election.
Police are also considering four alleged breaches of the Electoral Act related to social media content on election day, including two by the Opportunities Party's communications director, Sean Plunket.
One involved Television New Zealand rebroadcasting its Te Karere programme on election day.
The 38 alleged dual voters compare to 126 in 2014, when 45 written warnings were issued.
A spokesperson said more than half of such cases involved people who voted in advance and then again on election day.
"Dual votes are identified during the scrutiny of the rolls process after election day."
"This is when all of the electoral rolls are scanned and checked to compile a list of all people who have voted," she said.
"The process allows the commission to identify voters who have voted more than once. If it appears that any voter has received more than one ballot paper their ballot papers are extracted from the count and their votes are disallowed from the official results."
The maximum penalty is two years in jail and a fine of $40,000.
The Electoral Commission said a small number of complaints in the lead-up to the election required investigation by the commission, including election advertising, treating, bribery and donations.
Police said they were unable to identify a person who was allegedly going door-to-door in Auckland's North Shore, bribing people to vote National.