The blunder involved the bank allowing for an overdraft facility believed to be $10m to be credited to a business account, though the account holders are thought to have applied for only $10,000.
A substantial part of the sum is missing and the couple have fled.
Police launched an investigation after being contacted by the bank.
The service station, known as BP Barnett, was run by Huan Di Zhang and Hui Gao. It was recently placed in receivership.
No one seems to be clear on where the couple have gone, but the Rotorua Review newspaper said they might be in China.
A source told the newspaper a police liaison officer was sent to China recently to search for the couple. It understood $6m was missing, with $4m recovered.
A Westpac spokesman today refused to confirm the amount mistakenly advanced to the account, but said the bank was "pursing vigorous criminal and civil action to recover the sum of money stolen".
He said human error was responsible for the mistaken windfall, not a systems error, and that the bank was reviewing its procedures.
Police said those being sought had fled overseas and international police liaison organisation Interpol had been called in to help find them.
Officer-in-charge, Detective Senior Sergeant David Harvey of Rotorua CIB, would not confirm today the names of those being sought, or the amount of money in question.
The service station was left abandoned with stock on the shelves.
Neighbours said it had closed suddenly and for a while motorists were still driving in to fill up thinking it was open.
Signage was only being removed from the building today.
Rotorua's The Daily Post newspaper reported that the company owed money to creditors.
The creditors report, being prepared by Corporate Finance Limited, is not due until 19 July. Corporate Finance Limited would not comment.
Banking ombudsman Liz Brown told The Daily Post that generally speaking it was a criminal offence for people to spend money accidentally put into their bank account if they knew the money wasn't theirs.
In her 15 years as banking ombudsman she had been involved in 10 to 20 cases of this nature. They were legally referred to as "payment by mistake".
