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.



"I take responsibility for that, the tweet was sent in error and deleted within seconds and it was reported as soon as possible to the returning officer," he said.



It was a draft which he sent unintentionally and deleted immediately before reporting it to his campaign who he said reported it to the returning officer.



He was not aware of the rules at the time it was sent but said no one else pointed out his mistake to him before he deleted it.



He would co-operate with the Electoral Commission if it investigated the tweet.



A spokeswoman the commission said yesterday they had not received a complaint about the post and it had not previously been brought to their attention.



The Commission was now looking into the matter and could ask to discuss the matter with Cunliffe before any decision was made.



Guidelines about election coverage which were sent out to by the commission last week said the ban on campaigning on election day included any statement that was likely to influence a voter to vote or not to vote for any candidate, or to abstain from voting.



The commission said that when it became aware of breaches it would look into the incident and where appropriate refer the matter to the police.



Williams easily won the election in the seat held by Labour since 1922. Her margin over National's Matthew Doocey was 4613 votes.