Labour leader David Cunliffe might have fallen foul of the law with a message posted on his Twitter profile on the day of the Christchurch East by-election.
The Electoral Commission is looking into the tweet by Cunliffe on Saturday urging Christchurch East residents to get out and vote for the successful Labour candidate Poto Williams.
"If you are resident in Christchurch East don't forget to vote today - for Labour and Poto Williams!" he wrote.
Under Electoral Commission rules, no campaigning of any kind is allowed on election day.
The message has since been deleted. Cunliffe yesterday took responsibility for the tweet, saying it was sent in error but was reluctant to explain how. It was "deleted within seconds".
He said he was not aware of the rules at the time it was sent. He would co-operate with the Electoral Commission if it investigated.
Williams easily won the election in the seat by a margin of 4613 votes over National's Matthew Doocey.
- Fairfax Media
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