The Serious Fraud Office will start trawling through ASB records to decide whether an investment adviser, fired for fraud over advice he gave to customers involving millions of dollars, will face criminal charges.
The alleged fraud involved an employee, who had worked for the bank for more than 10 years,
making fictitious investments on behalf of 22 customers.
The customers have been identified and had their money returned where irregularities have been confirmed.
The ASB, which will not say exactly how much the employee defrauded or what branch he worked for, have called in the SFO to investigate.
SFO acting director Gib Beattie said a team of financial investigators - who will interview witnesses and others involved - and forensic accountants, who trace the money records, would work under the direction of a SFO-employed lawyer to investigate the case.
There was no knowing how long it would take, Mr Beattie said.
"When you get cases in like this, you're required to go away and investigate the matter thoroughly so that you actually get to the bottom of the offending - you determine who was involved, how many people were involved, where the money might have gone. All those sorts of issues take time to work through."
The case would then be reviewed by the SFO director who would make sure there was sufficient evidence and decide whether charges would be laid.
If he decided to prosecute, the police would be called to arrest the former employee.
"We will do everything we can to expedite the case but these things invariably take time to put the investigation together and complete it," Mr Beattie said.
ASB said the dismissed employee acted alone.
The bank would continue to work with customers "to ensure that no customer is adversely affected".
"The fraud was elaborate and involved the investment adviser making fictitious investments on behalf of customers," the company said in a statement.
The case is unrelated to one reported on Monday of a 67-year-old South Island man who was stopped from leaving the country after allegedly committing a multimillion-dollar fraud involving "advanced fee" payments.
