ACT Party leader John Banks says Labour leader David Shearer should resign for failing to tell Parliament he had more than $50,000 in a New York bank account.
Mr Shearer has failed to declare the bank account over the past four years, despite being required to do under Parliament's rules.
Since becoming an MP in 2009 Mr Shearer has declared income from the account to Inland Revenue.
He said on Tuesday he had no explanation for the oversight but he has been honest about his mistake and has not tried to hide it.
Mr Banks told Radio New Zealand's Morning Report programme that Mr Shearer's explanation is hard to believe.
"He didn't suffer a memory lapse with the IRD for the last four years, he remembered he had the secret money for four years when he filled out his IRD form, he tells us all."
The ACT leader said Mr Shearer called on him to resign last year over forgetting Kim Dotcom's donations to his mayoral campaign, and Mr Shearer should now abide by that standard.
Mr Shearer has amended the MPs' Register of Pecuniary Interests to include the New York-based account in which his United Nations pay used to be deposited.