Police in Sydney have arrested a woman as part of an ongoing investigation into a multi-million-dollar fraud against the National Australia Bank (NAB).

Key points: Police arrested Helen Rosamond as part of an investigation into allegations of corrupt commissions at NAB

Police arrested Helen Rosamond as part of an investigation into allegations of corrupt commissions at NAB Authorities said the commissions were valued at about $40 million over five years

Authorities said the commissions were valued at about $40 million over five years The alleged fraud was reported by a whistle blower but no-one at the bank was under investigation, NAB said

In February 2018, detectives from NSW Police's Financial Crimes Squad established Strike Force Napthali to investigate allegations of corrupt commissions being paid for contracts with the bank.

Police claim those commissions were valued at about $40 million over a five-year period.

The woman arrested is Helen Rosamond, a director of events management company Human Group — a former supplier to NAB.

Ms Rosamond will be charged with 56 counts of bribing an agent and two counts of gaining benefit by deception amounting to $6 million, police have confirmed.

The relationship between the bank and the company goes back 12 years, but police allege the offending took place between 2013 and 2017.

Ms Rosamond was arrested at a home at Potts Point this morning and has been taken to Kings Cross Police Station for questioning.

She appeared in the Sydney Central Local Court this afternoon via audio visual link and was granted strict conditional bail.

Bail had been opposed by the prosecution on the grounds there was a risk Ms Rosamond would fail to appear, and could interfere with evidence and witnesses.

It argued the case against her was strong and a jail sentence was likely.

In considering bail, Magistrate Robert Williams noted the majority of the offences carried a jail term of up to seven years.

He said the facts showed it was clear there was "a sophisticated operation embarked upon with Ms Rosumond's co-accused" and that the prosecution case appeared strong.

However he granted bail on several strict conditions, including surrendering her Australian and British passports and those of her two nine-year-old children.

She will also have to report to police daily, abide by a night-time curfew and put up a surety of $200,000.

Due to its complexity, the case will not be ready to begin for at least 18 months, the court heard.

'NAB is the victim'

Acting Assistant Commissioner Stuart Smith would not rule out further arrests, and said it was the first time he had seen senior executives involved in a fraud.

NSW Police seized documents and electronic storage devices during raids in April. ( Supplied: NSW Police )

"We will allege there are very senior positions involved," he said.

"There are those in charge of carrying out business with integrity and it is very clear this hasn't occurred in this investigation.

"The involvement of CEOS and senior executives is something we haven't seen before."

A spokesman for NAB said no one at the bank was under investigation, including former CEO Andrew Thorburn, who resigned in the wake of the banking royal commission.

"The alleged fraud was reported by a whistle blower. NAB responded immediately, investigated and reported it to police," the statement said.

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

"We continue to cooperate fully with police and we thank them for their important and work in investigating this matter.

"NAB is the victim in this matter. Police have confirmed that no one at NAB is under investigation, including former CEO Andrew Thorburn, and there is no suggestion of any wrongdoing by anyone at NAB.

"While this remains before the court it is not appropriate for us to comment further."

Last April, three businesses were raided in relation to the ongoing police investigation into fraud involving NAB and allegations of corrupt commissions being paid for contracts.

A remuneration report released in November also pointed to an investigation into an alleged fraud involving a former NAB employee.

The matters included "certain control failings and breaches of policy" in the office of the CEO, the report said.

It did not name Mr Thorburn's former chief of staff, Rosemary Rogers, but her activities have been scrutinised closely by police in NSW and Victoria.

No end date for investigation

Mr Smith said forensic accountants had spent "thousands of hours" investigating the fraud and that $10 million worth of material had been seized.

"We seized a warehouse for documents and electronic documents and hard drives and systems, and we are forensically working our way through," he said.

"There's been a lot of benefit by these individuals and we've done restraining orders on over $10 million worth of property.

"We've been at this for 12 months and we have begun the resolution phase, but I can't put an end date on the investigation."

A large number of documents were seized along with electronic storage devices at three Sydney businesses by detectives in April last year in relation to the investigation.