The Labour Party leader, David Shearer, faces accusations of hypocrisy for failing to declare a New York-based bank account with more than $50,000 in it.
Since becoming an MP in 2009 Mr Shearer has declared income from the account to Inland Revenue - yet has not included it in his annual declaration of pecuniary interest.
Mr Shearer has now amended the MPs' Register of Pecuniary Interests to include the account.
He rejects any suggestion of hypocrisy, saying he's been honest about his mistake.
MPs must declare bank accounts with more than $50,000 in them as well as disclosing any business interests, shares or properties, and any gifts they have received.
Mr Shearer said on Tuesday he had no explanation for the oversight.
He said he had included all of his interests in New Zealand but had forgotten about a New York-based account in which his UN pay used to be deposited.
Mr Shearer said he has been honest about his mistake and has not tried to hide it.
Prime Minister John Key has suggested Mr Shearer is guilty of double standards.
"You don't get cut any slack from the Labour Party when you say (you've made) a mistake but when they make one they don't want anyone to have a look at it."
Mr Key said Mr Shearer's failure to disclose the bank account earlier looks like a brain fade.
ACT Party leader John Banks is demanding Mr Shearer stand down because he is on record saying those who suffer from a memory lapse are not fit to hold office.
He says the Labour leader's hypocrisy is staggering and his behaviour is appalling for for someone who claims to have such high ethical standards.
Mr Banks says Mr Shearer is a hypocrite and should resign.