Mr Thorburn and NAB chairman Ken Henry suddenly resigned on Thursday in the wake of criticism from the banking royal commission of both men.

On Wednesday, The Age and the Sydney Morning Herald revealed police are investigating whether shoddy oversight inside Mr Thorburn's office enabled and emboldened Ms Rogers and the event contractor to pull off a suspected multi-million dollar fraud.

In December, both papers revealed how Mr Thorburn took a luxury Fiji holiday and a Thermomix arranged by Ms Rogers and the Human Group. While Mr Thorburn paid several thousands dollars towards the trip, the full cost of it and the Thermomix was ultimately passed on to NAB shareholders via inflated invoices issued by the Human Group.

NAB disclosed a small number of "inadvertent" breaches of the company's policies by Mr Thorburn in relation to Ms Rogers activities in its annual report but said he had been cleared of wrongdoing by the bank's board. According to October's board minutes, NAB is of the belief the mis-appropriation of funds had begun occurring years before Mr Thorburn's appointment as CEO.

Ms Rogers was the chief of staff of Mr Thorburn’s predecessor Cameron Clyne, who retired as the bank’s boss in August 2014. There is no suggestion Mr Thorburn is involved in the alleged criminal conduct.

Proceeds of crime restraining orders were slapped over Rosemary Rogers $6.7 million property portfolio amassed between 2013 and 2017, the $1 million cheque, two speedboats and Ms Roges’ 2015 Range Rover in December.

Under the orders, Ms Rogers’ husband Anthony must provide the police with a list of any property held in his name. Calls to their family home in Williamstown went unanswered on Thursday.

Four properties purchased by Ms Rogers are subject to the freezing orders.

This includes a sprawling property in Bellbrae, a desirable rural hamlet nestled between popular beachside towns Torquay and Anglesea. The Rogers’ purchased the property in 2013 for just under a $1m but it is expected to have increased in value.

The Rogers' elegant four-bedroom, heritage-fronted home in Williamstown, replete with an indoor pool is also frozen. The couple purchased it December 6, 2017 - just weeks after the banking royal commission was called by the federal government.

Also frozen is a two bedroom $700,000 plus “executive” apartment in Williamstown with bayside views purchased in December 2017 as well as a large flat in the Dimmey’s apartment redevelopment in Cremorne just a few kilometres from NAB’s Docklands offices.

A 2016 Chaparral Sunesta motorboat and a Anglapro Sniper motorboats have also been frozen.

Ms Rogers' overseas jaunt is the most egregious transaction being probed by NSW detectives, according to NAB sources.

Two NAB insiders have said that they both had concerns about a culture of largesse and extravagant spending that became normalised inside Mr Thorburn’s office and which allegedly centred around Ms Rogers and contractor, the Human Group.

One NAB insider alleged that Ms Rogers kept a “slush fund” from which she gave staff benefits, including expensive gifts.

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The police suspect Ms Rogers and the Human Group devised a system in which they allegedly issued inflated invoices to the NAB to collect kickbacks and cover certain private expenses, including Ms Rogers' $500,000 overseas holiday.

A NAB spokesman said the bank continued to co-operate with police on an inquiry which was instigated by a whistleblower report.

"If the alleged fraud is proven, it represents a most serious breach of trust by a former employee," the spokesman said.