New Zealand First leader Winston Peters says there is nothing unusual about a non-disclosure document party board members are being asked to sign.
The document, in which board members agree to keep party business and membership confidential and not to talk to the media, also requires them not to associate with any other political party for three months after the next general election.
Peters said such agreements were common in every political party.
"Members who join the board are required to give an undertaking to keep the business confidential, there is nothing untoward in that at all."
He understood it had previously been in the nomination form but had been made into a separate document.
"I believe, internationally, it's common that if you're going to apply for the job that you're going to give an undertaking that you'll keep the business confidential to the meeting."
Of the three-month non-association clause, Peters said: "If you're not in it for the long-haul, we're not interested in putting people on the board."
"We have not survived all these years, as the second-longest surviving party in this country's history with one name … by being careless about what is board's business."
Despite the agreement spelling out that the agreement was a contract enforceable in the courts, Peters said there was no penalty for breaching the terms.
"There's no penalty, we just expect people to have the level of honesty to give a commitment," he said.
Spokespeople for Act and the Green Party said neither asked board members to sign similar documents.
A Green Party spokeswoman said: "There are no written contracts for members of our party executive or policy committees. Under the Privacy Act the Green Party does require officeholders (and staff) to sign agreements specifically to keep the personal details of members private."
"ACT doesn't make its board sign anything like this," a spokesman said.
But a National Party spokesman said: "Like many boards, the members of the National Party board sign a code of conduct agreement, which includes provisions for confidentiality."
National's rules prevented membership of more than one party at a time but did not extend to a three-month non-association clause.
The Labour Party did not immediately respond to a request for comment.