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.