A Brisbane woman is taking on one of the nation's biggest banks in the Supreme Court, alleging its deceptive conduct has left her financially ruined.

Key points: Leith Williamson alleges NAB allowed her former partner to secure loans in her name

Leith Williamson alleges NAB allowed her former partner to secure loans in her name The loans are worth hundreds of thousands of dollars

The loans are worth hundreds of thousands of dollars Ms Williamson says she was not aware of the loans

Leith Williamson alleges the National Australia Bank (NAB) allowed her former partner to secure hundreds of thousands of dollars in loans in her name, as well as a bank guarantee using her home as security, without her knowledge.

The 58-year-old marketing consultant has spent four years trying to work out how debts approved by NAB for her former partner, real estate agent Phillip Waight, spiralled out of control.

In its statement of defence, the bank said it had not been able to find documents which contained a guarantee signed by Ms Williamson for a loan worth $320,000.

Ms Williamson said that was because the guarantee did not exist.

"Over an eight-month period, I was given so many excuses — 'they're in the mail, they're coming from a storage area' — by phone and by email I was given so many different excuses," Ms Williamson said.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 10 minutes 3 seconds 10 m Brisbane woman takes on one of the nation's biggest banks, alleging deceptive conduct

In its defence submitted to the Brisbane Supreme Court, lawyers for NAB conceded that key paperwork could not be found.

"On a date that cannot be presently particularised as the documents cannot be located, Ms Williamson provided a guarantee … to Mr Waight … and signed an acknowledgement in her capacity as a guarantor for $320,000," the bank said.

NAB refers to other documents in its defence, which it said showed Ms Williamson was aware of all the financial decisions relating to her joint account with Mr Waight at all times.

But Ms Williamson said the documents sent to her prior to the case were fraudulent.

"The documents that they have presented me with I've never seen before, I've certainly never signed and I knew nothing about," Ms Williamson said.

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Bank to 'continue to defend' case

NAB declined the ABC's request for an interview, but released a statement.

"As the matter is presently before the courts and to respect the privacy of all parties we are unable to comment on the specifics of the matter," the bank told the ABC.

"However NAB will continue to defend this matter. As always, NAB is willing to meet with the customer to discuss this matter."

The Brisbane apartment Dr John Williamson is allegedly the guarantor for. ( ABC News )

Ms Williamson's former partner Phillip Waight told the ABC the case was really against NAB, and he was just listed as a second defendant.

Mr Waight said Ms Williamson was fully aware of the couple's finances at all times, and everything was above board, but he would not be interviewed.

In another twist to the case, Ms Williamson's elderly father, retired surgeon John Williamson, appears in some bank documents as the guarantor of the purchase of a luxury apartment under the names of Ms Williamson and Mr Waight.

Mr Williamson held one account with NAB, but said he was never involved in the purchase of the property.

However, he is listed as guarantor on a lending submission that Ms Williamson obtained after her case was examined by the Financial Ombudsman.

"It just indicates for my part, that the bank had used my details and my privacy to do what they wanted to do," Dr Williamson said.

Banks have 'bottomless pit of resources' to fight cases

Sydney University political economist Evan Jones has helped dozens of people with their grievances against banks.

"The past history is pretty depressing really," he said. "Most of the time, the banks have all the resources, and a bottomless pit of resources, and they'll pursue a victim to the last drop of their blood, basically."

Dr Jones has been supporting Ms Williamson and believes she has a strong case.

"She's done her homework, and as I keep emphasising, she has incriminating documentation: the NAB has failed to come up with anything substantial that runs contrary," Dr Jones said.

"The crucial element is they claimed she signed a guarantee, [but] they have no documentation, no documentation whatsoever."

Ms Williamson's case alleges the bank broke Australian Consumer Law by engaging in misleading and deceptive conduct when it allowed the lending to continue without her knowledge.

The case also argues the bank has failed to adhere to the Code of Banking Practice, because it did not behave as a "prudent lender".

Previous judgment said banking code 'legally binding'

The legally binding nature of the code has already been tested in court.

Lawyer Grant Walker from Madgwicks took NAB to court after one of his clients signed a loan guarantee that was not properly explained to him.

Mr Walker's team won the case, but NAB is appealing the judgment.

"It was an acknowledgement by the court that the Code of Banking Practice is legally binding, and that the obligations that the banks say that they will live up to, the court has now acknowledged that they should do that," Mr Walker told the ABC.

"This decision just shows that the consumers have one more string to their bow in ensuring the banks live up to those promises.

"It's still incredibly difficult for consumers to take on the banks in litigation; you couldn't pick a worse litigant I don't think."

Dr Jones said that previous court case was an important decision for Ms Williamson's case.

"Leith can leverage her specifics on the basis of those general propositions," he said.