An apologetic letter from the bank explaining the error and its intention to take the money back arrived too late on Saturday.
"You read about these things every now and then. You would think by now they would stop putting money into accounts," Mr Downer said.
"It's a major, major inconvenience that makes me look like an absolute arse, as well as really stressed us out for a wee while."
Mr Downer said he was far from a runaway millionaire and had made every effort to work out what was going on with his account, contacting ASB several times.
He did not realise the bank's mistake, because he had himself transferred thousands of dollars into the same account on the same day.
That was done so the business could purchase vehicles for its fleet.
When ASB tried to retrieve their money a few days later there was insufficient money available, and the account was charged a $20 fee.
Mr Downer said that when he noticed the fee he rang his bank manager, who was unable to confirm why the fee had been charged.
Later in the month someone from the bank's merchant division called the business asking to speak to Mr Downer.
He was unavailable but they would not leave a name or contact number, Mr Downer said, only vaguely explaining they would need to withdraw $6000, and would send a letter.
Suspecting the caller was a fraudster, Mr Downer phoned ASB and reported the conversations.
He heard nothing more until the owed money was removed from his bank account on Friday, putting it into the red.
Mr Downer said he had received nothing but good service from ASB until this incident.
He had no issue with the need to repay the money, but was disappointed with the way the bank had handled its mistake.
A visit to the bank yesterday finally resolved the situation and Mr Downer now has access to funds to pay his bills.
The bank will also refund any related penalty costs.
Greg Beehre, ASB's general manager of global transactions, said they became aware of a technical error last month "caused by an external vendor that resulted in the duplication of a number of merchant settlement deposits".
Mr Beehre said the error was fixed immediately and most payments were reversed with no inconvenience to customers.
"For the small number of duplicated payments that were still outstanding, we contacted the affected customers to let them know that we would be performing a debit on their account to recover the duplicated funds.
"We cannot comment on the specifics of any of our customers but we do apologise to Mr Downer and any other customers impacted by this issue for any frustration and inconvenience caused."
Other bank cases
*Auckland man Mehdi Soheili spent more than $100,000 in a week after his bank accidentally transferred $141,991 into his account. Soheili's lawyer said he had a gambling problem and his spending last November was a crime of opportunity.
*Rotorua man Leo Gao served 16 months of a four-year and seven- month sentence after fleeing overseas when his account accidentally received $10 million.
*Kara Hurring, Gao's then-partner, joined him in Macau. Gao, nicknamed "the runaway millionaire, was released on parole in December last year.
