The Electoral Commission is reviewing complaints about breaches the Electoral Act - including tweets by prominent sportsmen - but a legal expert says prosecutions are unlikely.

The Electoral Act 1993 forbids broadcasting of messages on Election Day which might influence other voters.

But many potential breaches were reported on social media, including tweets from Jonah Lomu, Israel Dagg, Eric Murray and Hamish Bond tweeted in support of John Key on Saturday.

Mr Dagg's tweet read: "Just voted for @johnkeypm and the National party all the best for tonight #blueallday #National." Dagg later replaced his tweet with an apology saying he "doesn't read the fine print", which has also since been deleted.

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Murray and Bond tweeted their support for Key, but Murray later said he didn't realise it was against the law, and deleted the post.

Mr Lomu tweeted: "@johnkeypm All the best for Tonight Get in there everyone your last chance to vote and grow NZ Go "National"#vote2014nz #vote2014nz." The post has since been deleted.

John Key told media yesterday that it was "fantastic to have their support".

"They possibly know a little more around rugby than electoral laws, but I hope they don't get into too trouble for it."

Electoral Commission spokeswoman Anastasia Turnbull said the accounts of Dagg, Lomu and a fake Richie McCaw account had all prompted complaints.

Vexatious complaints and those related to scrutineers and door-knockers were also taking time to sift through before the number of legitimate complaints could be worked out, she said.

Sixty complaints were handed to police by the Electoral Commission in 2011, but none resulted in prosecution, said law professor Andrew Geddis of University of Otago.

While offenders can be fined up to $20,000, Mr Geddis said that was highly unlikely.

The top penalty would be for "someone who goes out with a concentrated plan to flout the rules or sway the election".

"Sports stars won't have the book thrown at them."

Mr Geddis said the Act could be considered toothless "in the sense that no one's had to go before the court to answer to it".

But passing on complaints to police could be seen as a deterrent, Mr Geddis said. "The NZRFU wouldn't be happy with Israel Dagg being phoned by the police."

Mr Geddis noted that unapproved TV footage of party leaders casting votes would also breach the act.

Political parties, MPs and the Young Nats temporarily deactivated or refused to update their Facebook pages Saturday, as encouraged by the Electoral Commission.

Chief Electoral Officer Robert Peden said the decision to refer an apparent breach to police was made on a case by case basis. The commission took into consideration whether a post, tweet or page was removed once it had been identified, the scale of the breach, and the size of the potential audience of the post.