The Electoral Commission has referred Labour Party leader David Cunliffe to police for posting a statement on Twitter the day of the Christchurch East by-election.
Mr Cunliffe tweeted on Saturday encouraging people to vote for the Labour candidate Poto Williams, but removed it soon after posting it.
He said on Tuesday he took responsibility for the tweet and that he has written it himself.
The commission said on Wednesday it believes that the tweet breached the Electoral Act because it was a statement published on polling day advising, or intended, or likely to influence electors as to who they should vote or not vote for.
If convicted, Mr Cunliffe would be liable for a maximum fine of $20,000.
Mr Cunliffe said this is a routine part of the commission's process and he would fully co-operate with any inquiry.
Poto Williams won the by-election with 8119 votes - more than double those of her nearest rival - Matthew Doocey from the National Party.