The Electoral Commission has referred 126 voters to the police for voting twice in the General Election - double the number of the 2011 election.

The commission said most of those believed to have voted twice had cast an advance vote and then voted again on election day.

Voting more than once carries a penalty of a jail term of up to two years and/or fine of up to $40,000 or up to $10,000 if the court decides it was at the lower end of the scale and not a deliberate attempt to manipulate the election result.

Under the Electoral Act 'personation' is considered a corrupt practice, including voting as another person or voting more than once.

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However, police have discretion over whether to prosecute and tend to issue warnings to first time offenders. Of the 63 dual voters in 2011 32 were given warnings and 29 were cleared. Only two remained unresolved.

A spokesman for police said the practice was for warnings to be issued for first time offenders and in the past only those who had previously been warned were prosecuted or if it was clearly deliberate.

Labour's Justice spokeswoman Jacinda Ardern said it was concerning the numbers of dual voters were so high and it appeared to be linked to the increased use in advance voting in 2014.

"If there is any part of the process that needs to be tweaked to ensure that's not happening, then we should be looking at that."

She said it was something she expected the Justice and Electoral Select Committee would consider as part of its regular inquiry into an election.

The Electoral Commission earlier referred two cases to police of people who posted on social media that they intended to vote more than once. It is not known whether those two are also included in the referrals of dual voters.

So far the Electoral Commission has referred a total of 156 breaches to police, including 26 people alleged to have broken strict no advertising rules on social media on election day. Those included Jonah Lomu, Israel Dagg and Eric Murray who tweeted support for Prime Minister John Key.

Two others had shared a Facebook message on National MP Nikki Kaye's Facebook page.

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The 26 referred for social media breaches compared to just five in 2011.

Other referrals included a George FM host for comments made on radio on election day, and two candidates and a community group who published advertisements without promoter statements.

The Electoral Commission has called for reform so that low level electoral offences are dealt with by other means than the Police because of long delays and low prosecution rates. That recommendation was rejected by the Government alongside other suggested reforms from the MMP Review following the 2011 election.

According to police figures, of the 93 total referrals for suspected electoral law breaches in 2011, 85 got off with a warning and none have yet resulted in a prosecution although five were still unresolved.

Police reported that for the 2008 election there were 88 referrals of which seven were prosecuted - two of those were fined for late candidate returns, four were given warnings by the court and one was given police diversion.