The Serious Fraud Office has taken court action against Jami-Lee Ross to prevent him from publishing legally sensitive documents.
The independent MP is facing criminal charges, laid by the SFO in January, alongside brothers and businessmen Shijia (Colin) Zheng and Hengjia (Joe) Zheng, and New Zealand Order of Merit recipient Yikun Zhang.
The charges stem from allegations relating to two $100,000 National Party donations. All four men have denied the charges and a trial in the High Court at Auckland is due to be held in September next year.
However, Newstalk ZB and the Herald have learned the SFO sought an interim injunction against Ross, who now leads the Advance New Zealand Party.
The injunction application was filed last Friday in the High Court at Auckland.
It is understood the SFO inadvertently disclosed material to the defendants in the case as part of its investigation and one party indicated an intention to publish the information.
In a statement this afternoon, the SFO "welcomed a court decision" confirming the confidentiality of material that was "inadvertently disclosed" by the agency to the defendants.
"The material was disclosed recently during the course of the agency's compliance with its normal disclosure obligations," the statement said.
"The SFO acted with an abundance of care in seeking the court order as one of the parties had reportedly expressed an interest in publishing the material."
The SFO said it believes any publication of the material would have breached the SFO's secrecy provisions and been contrary to requirements of confidentiality applying to the use of material obtained through court proceedings.
"However, the SFO sought a court order to ensure there was no doubt that the material remained confidential," the statement read.
Ross confirmed urgent orders had now been granted against him by the High Court.
"This is an appalling attack on free speech," he told NZME. "I am now having to take legal advice on my rights to free speech."
Ross said he "remained concerned and convinced" the National Party's donations "do not stack up to scrutiny".
"While I had no intention of breaching peoples privacy - I did want to reveal the nature of the issues it revealed as a matter of public interest," Ross said in a tweet after the Herald's story was first published.
Last Wednesday in the House of Representatives, however, Ross used his parliamentary privilege to discuss the SFO material which he claimed was sent to him "in error".
"I have before me here, 60,000-odd line items of donations that are from the National Party," he said.
"It was given to me in error – I call it a leak. It's the type of information that only party secretaries and chief financial officers and auditors tend to see. It's line by line individual information. It's the very information that people complained about in the inquiry that is not public enough."
Ross, who is the incumbent in the Botany seat, also said he "was threatened with injunctive action" if he attempted to table the document in the Parliament.
"But I was asked, extremely seriously, not to release this information because it is every single line item of donations for the National Party for the last couple of years," he said.
"In here there's hundreds of thousands of dollars linked to the National Party's Chinese group."
But Ross said he wasn't going to table the document and would "take the high road".
Tabling a document in Parliament makes it available to all MPs, however, it must first be checked by the House Clerk that it's not defamatory.
The SFO's investigation over the National Party donations was prompted by Ross going public with allegations against then-National leader Simon Bridges, which Bridges has adamantly denied.
Ross then laid a complaint with police, sparking the SFO inquiry and his criminal charges.
"I was the whistleblower. I still consider that I was right to raise the concerns," he told media after his first court appearance earlier this year. "There is no own goal."
Charging documents against the group allege two donations of $100,000 in 2017 and $100,050 in 2018 were made "in circumstances where the identity of the donor was not disclosed in the National Party's Annual Return of Party Donations".
"The defendants adopted a fraudulent device, trick, or stratagem whereby the 2018 donation was split into sums of money less than $15,000, and transferred into the bank accounts of eight people, before being paid to, and retained by, the National Party," the SFO alleges in the court papers.
The same allegation is made by the SFO for the 2017 donation.
The SFO is also conducting four other investigations over electoral funding allegations, which include a probe into the New Zealand First Foundation, Labour Party donations from 2017, and separate investigations into two mayors' expenses.
The SFO has previously said it will make a decision before September's general election on whether to lay charges in relation to the NZ First Foundation, which has been bankrolling the NZ First Party.
The investigations over donations made for the Auckland and Christchurch mayoral elections came from separate referrals from police about Auckland Mayor Phil Goff's election expenses and Christchurch Mayor Lianne Dalziel's expenses.
The SFO has remained tight-lipped over details relating to the Labour Party investigation.