Instead, it put $10 million into his account. Now, he is the subject of an international manhunt.
And the Herald understands he may have had a head-start on authorities.
The whereabouts of Mr Zhang, who owns 60 per cent of the BP Barnetts business, is unknown.
The money was in the account on or about May 5. The following day, the BP station closed its doors. But it was only on Thursday this week that police said they were investigating.
In the meantime, Westpac hired a private investigator - Mike Dingwall, who describes himself on his business card as a "former police licensed investigator" - to conduct inquiries.
Police would not say yesterday when Westpac alerted them to the missing money.
Inquiry head Detective Senior Sergeant David Harvey of Rotorua CIB said police had called on Interpol to help find "individuals associated with the account" who were believed to have fled the country with a large sum of money "mistakenly advanced" by Westpac.
It is understood as much as $6 million is missing. The bank has recovered $4 million.
Westpac spokesman Craig Dowling refused to confirm the amount mistakenly handed over, saying only that the bank was "pursuing vigorous criminal and civil action to recover the sum of money stolen".
The bank also refused to reveal how the money was removed from the account, saying only that human error, not a systems error, was responsible for the windfall and that the bank was reviewing its procedures.
Police refused to answer questions yesterday and released only scant information.
However, a neighbour said she understood Ms Young was with her mother in Blenheim and had not fled NZ.
Chevi Lambert, manager of Andy's Cellar close to the BP, said Gao phoned his girlfriend and told her to get out of town.
"I don't know when the money went out into his account, but I know he did a runner on the Wednesday."
Ms Lambert said Mr Dingwall had visited her shop with a photo of Ms Young, asking if staff recognised her.
"And our answer was, 'Yes, it's Leo's girlfriend'."
Mr Dingwall told staff he had proof Gao had left the country and that records showed Ms Young had used his credit card in Auckland on Wednesday, May 6.
"That's the reason they thought she was involved ... because she had tried to use Leo's credit card."
One report last night suggested a police liaison officer had been sent to China to search for the missing millions and those behind its disappearance.
Tania Davis, director of ZeroTo100Automotive which adjoins the BP, knew the business was in real strife when she saw a receivership notice left at the station.
Banking Ombudsman Liz Brown said that it was generally a criminal offence for people to spend money accidentally put into their account if they knew it wasn't theirs. In her 15 years in the role she had been involved in 10 to 20 cases of this nature.
Finance workers' union Finsec said the bank worker responsible for a Rotorua couple being given $10 million in error was unlikely to be sacked.
"It's not uncommon in the day-to-day business of data entry in a bank for mistakes to be made," Finsec spokesman Andrew Campbell said.
"The general rule in relation to negligence is that if there's not intention, assuming there wasn't, then that's normally not gross misconduct, so generally wouldn't be a dismissible offence."
Mr Campbell said several factors would be taken into account, such as the worker's previous record at the bank.
"There's no black and white answer," he said.
"If this person does end up being a Finsec member and we're asked to represent them, it's the sort of thing that, on the face of it, there'd be a reasonable degree of mitigating evidence."
- additional reporting: NZPA
EASY COME, EASY TO SEND OVERSEAS
The international transfer of as much as $10 million could have been as easy as walking into a bank, setting up an account and making one simple transaction. "There's no particular obstacle to doing it at all," said Dr David Tripe, head of Massey University's Centre for Banking Studies.
However, he said it was likely Leo Gao would have made up a story to explain the transfer of up to $10 million either as one transaction or in several increments.
"I would be surprised if some bank staff member hadn't looked at it ... I would suspect they would have had a story to make it look plausible.
"It may have been broken up into chunks of $500,000, in which case it would raise much less [concern]."
Dr Tripe said standard international procedure for large transactions ($250,000 or more) meant that even if the money had been transferred in stages, it would have been subject to a "value dating process", which would mean a delay of up to two days before funds became available.
Experts believe there is little doubt keeping the money would be illegal. Massey University senior banking lecturer Claire Matthews said if a person "reasonably believed" money mistakenly put into their account was theirs and had spent it, they would not have to pay it back.
"If a few hundred dollars ended up in your account and you weren't sure what it was but you figured, 'Oh well, it's just one of those things that was due to come in', and you went out and bought some new clothes or whatever, that would be okay," she said. "But if you never got anything other than your weekly wage and all of a sudden a few thousand went in and you spent it - then no."
Mrs Matthews said banks could pursue people for the debt. Rotorua police are investigating the matter as theft or fraud.
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