Helen Mary Rosamond, 48, falsified and inflated invoices from her event management company Human Group, swindling more than $19 million from the National Australia Bank between 2013 and 2018.
A jury found Rosamond guilty of 90 fraud charges in November, finding she gave corrupt benefits to Rosemary Rogers, former chief-of-staff to NAB chief executives, and obtained or attempted to obtain financial advantage by deception.
"The overarching motivation of the offender is accurately described as one of greed, self-gratification and enrichment," Judge Robert Sutherland said on Thursday, sentencing Rosamond to 15 years in prison with a non-parole period of eight years.
Successfully hiding her derived financial benefit from her own company and even her co-offender, Rosamond defrauded the bank for almost five years, a testament to her level of planning, the judge said.
Rogers pleaded guilty to corruptly receiving benefits for her role in the same scheme in February 2020 and is serving a maximum jail sentence of eight years with a non-parole period of four years and four months.
The judge rejected submissions seeking to place greater blame on Rosamond's co-offender.
The pair clearly depended on each other to conduct the fraud, for which they were equally culpable, the judge said.
Rogers received a house, a BMW, boat, holidays and other benefits, Rosamond's trial heard.
Human Group began conducting legitimate work for NAB in 2006, before starting to defraud it.
It received more than $118 million in a 12-year period, with $54 million paid in the offending period from 2013 until early 2018.
Project codes for legitimate NAB business were used in Human Group's accounting software for fraudulent transactions.
The money was then drawn down to become the company's revenue.
"Project names, on some occasions, indicated the illegitimate use of the money," the judge said.
Extravagant birthday parties for Rogers' family members were processed under "Project Outlaw".
Rogers would ensure contracts were extended with Human Group and pay its inflated invoices, with a delegated authority to approve spending up to $20 million.
The pair continued their fraudulent activities over such an extended period, spending extravagantly, facilitated by that authorisation.
When a new CEO took over, they did not know Rogers had such authority.
The judge said such a large authorisation exemplified the level of influence and trust Rogers enjoyed within the bank, which she abused.
Rosamond had not breached the trust placed in her at the same level, but engaged in repeated and ongoing deception of her own company.
Her claim she wanted to express remorse, which came through "an apparently unquestioning third-party", was treated with "considerable reserve" by the judge, as Rosamond herself gave no evidence.
She will be eligible for release in December 2030.
NAB legal and commercial services group executive Sharon Cook acknowledged the bank's prior failures in not having sufficient controls and processes in place in a statement following Rosamond's sentence.
"After we became aware of the fraud, we overhauled our controls and processes, including reducing expense delegations and introducing additional checks on expenses," Ms Cook said.
"The issue was first reported to us by a whistleblower and NAB thanks them for coming forward and alerting us to the fraud."
Rogers resigned following the whistleblower report. Rosamond made another attempt to defraud the bank of more than $3.3 million.
She was also found guilty of trying to fraudulently obtain more than $500,000 by doctoring emails and inflating invoices for Christmas parties for retail property group Vicinity Centres.
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