Mr Key noted that the account's balance was above the $50,000 threshold above which it must be declared, "so it could be quite a bit more than that".
But Mr Shearer this morning again refused to say how much was in the account.
"I'm not going to go down that road."
"This is something between my family and myself, but it's over the threshold and as a result I've a duty to declare it."
Mr Shearer said he discovered the error because he was filling in his tax return at the same time as filling out his return for Parliament's register of pecuniary interests and "saw that I hadn't put it on there".
"Frankly I was horrified that I'd overlooked it and I moved straight away to correct it."
But Mr Shearer, who has previously criticised Mr Key for memory lapses in relation to GCSB briefings on Kim Dotcom and on which way he vote on alcohol reform, denied he was guilty of a double standard.
"When I myself found that (bank account) error I made the move to correct it, I didn't wait for anybody else to find it."
Mr Shearer indicated he'd omitted the bank account from the pecuniary interest form three times before he discovered his error last month.
"I understand people are obviously bemused by it. I had no excuse and I'm not trying to make an excuse. I wish had seen it sometime before."
Mr Shearer's 2012 pecuniary interest register entry included interests in two trusts, a house in Avondale, his family home in Pt Chevalier, and the section in Whananaki, Northland, where he spends holidays, as well as his UN pension scheme, parliamentary super scheme and a superannuation trust. He also disclosed a term deposit and a mortgage, as well as tickets for two to six Rugby World Cup games.
Since 2005, MPs have been required to disclose all their financial interests in the register each year, including debts, assets, such as property or shares; and gifts worth more than $500.
Late additions to the register because of oversights are not unusual - last year, National's Phil Heatley added a KiwiSaver account, Labour's Jacinda Ardern added tickets to a Tony Blair speaking event, and National's Chris Tremain added Rugby World Cup tickets.
However, some late entries are more controversial - last year Act leader John Banks belatedly declared a gift basket from Kim Dotcom he received while he was on holiday in Hong Kong. He had said he did not realise it was over the $500 limit.
Prime Minister John Key yesterday would not comment on Mr Shearer's omission, saying it was up to Mr Shearer. In 2008, Mr Key came under questioning about TranzRail shares his family trust had held in 2002 and 2003. Those shares were sold before the Register of Pecuniary Interests came into being in 2005.
Act leader John Banks, who was criticised by Mr Shearer for failing to remember a large political donation from Kim Dotcom, said the Labour leader "should apply his own ethical standards to himself and stand down".
"Shearer is on record as saying those who suffer from a memory lapse aren't fit to hold office," Mr Banks said.
Mr Banks said Mr Shearer didn't forget about the account just once, "he forgot four years in a row".
"Shearer's hypocrisy is staggering."
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"This is appalling behaviour for someone who claims to have such high ethical standards, let alone from someone who wants to be Prime Minister. If he's reckless with his own banks accounts, how can he be trusted with taxpayers' money?" Mr Banks said.
