Labour leader David Cunliffe and MPs Shane Jones and Grant Robertson could be forced to publicly declare who gave them donations of more than $500 during last year's leadership contest despite the party's attempts to keep them confidential.
The rules for the official register of MPs' financial interests require them to declare all gifts and donations of more than $500 other than donations for an election. Those returns are due by next Friday.
Sir Maarten Wevers, who oversees the Register of Pecuniary Interests, said the rules appeared to cover money received by the MPs in the Labour leadership primary.
"I would expect them to declare it because it is a donation for something other than an electoral campaign."
It could cause some concern for the three Labour MPs if they had assured donors their contributions would not be made public. The MPs would not have to disclose the value of donations, but would have to provide the donors' names.
Labour's rules for the contest stated that donations would be confidential, and the party has refused to release a list of them on those grounds.
A spokesman for Mr Cunliffe would not say whether the leader believed he should have to declare the donations, but said his register entry would contain all information that was required by the rules.
Mr Robertson also said he would stick to the rules but would not say whether that should include the donations. Mr Jones said he would discuss it with Sir Maarten next week. He would not say whether he received any help over the $500 threshold.
Sir Maarten said Labour's new United States primaries-style leadership campaigns, which had a fundraising element, were new to NZ and no specific consideration had yet been given to whether the Register of Pecuniary Interests should apply.
The rules for the register are decided on by a committee of MPs and set out in the standing orders that form Parliament's rules.
Labour's leadership contest rules did require contestants to declare donations of more than $1500 to the party's ruling council, but said they would not be publicly disclosed.
The donations did not have to be declared to the Electoral Commission because they were not for an election. In elections, candidates must disclose gifts of more than $1500 and parties must disclose more than $15,000.