Police have referred a complaint from former National MP Jami-Lee Ross about the party's election donations to the Serious Fraud Office (SFO).
Police confirmed the complaint, which was received in October last year, was in relation to the disclosure of political donations under the Electoral Act.
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The decision to refer the complaint to the SFO was made as it holds the appropriate mandate to look into matters raised by the investigation, police said in a statement.
In October last year, Mr Ross went to the police over allegations Simon Bridges was involved in unlawful activity relating to political donations.
He alleged Mr Bridges had committed electoral fraud by asking him to split up a $100,000 donation from businessman Zhang Yikun into smaller amounts, so they could be hidden from the Electoral Commission.
Mr Bridges, however, denied the allegations at the time, saying there was "zero chance of having done anything illegal I can tell you that".
Mr Ross released a recorded phone conversation between him and Mr Bridges, where he can be heard asking his then-leader what to do with the $100,000 donation.
"They talked to you about a $100,000 donation... What would you like done with it? It's currently sitting in a Botany electorate account," Mr Ross asked Mr Bridges in the phone conversation.
"Fantastic...Look, I just think we want it for the advertisements and the like, you know?" Mr Bridges replies.
The pair also talk about raising the issue of the donation with National Party President Peter Goodfellow.
On Tuesday, Mr Bridges said it was a "matter for the National party to fully cooperate with the SFO", but he had received no contact from the SFO.
He said it was "possible" Mr Ross could face prosecution, but remained adamant that National had "done nothing wrong".
"I've got nothing that concerns me about any of this... I don't think National MPs have anything to worry about. We have nothing to hide. We do want to see swift justice here."
Deputy National leader Paula Bennett was asked on Tuesday if she had seen a donation broken up, to which she replied "no".
"I know as much as you do."
National MP Gerry Brownlee hadn't heard of the referral when asked about it by reporters.
"Well I just heard about that. You just told me, so I don't have a view at the moment."
The SFO confirmed it had received the complaint.
"The Serious Fraud Office has received a referral from the Police about a complaint they received in October last year in relation to the disclosure of political donations under the Electoral Act," the statement said.
At the time, University of Otago law professor Andrew Geddis said the alleged offence carries a two-year prison sentence and would "lead to automatic explusion from Parliament".
"Essentially the allegation is that the leader of the Opposition Party committed a very serious offence under our electoral laws," he told Newshub.
"If these allegations are founded, Simon Bridges has to resign, full stop."
Neither police nor the SFO would make any further comment while the investigation is underway.
Mr Ross has been contacted for comment.