A woman who "promptly" paid all of her legal fees after a settlement with her ex is now being pursued for an extra $7800 by lawyers after a mistake was made when they billed her.

The woman was sent a bill for $52,000 but an error made by the cost assessor meant the figure didn't include GST, essentially giving Simone Harris a 15 per cent discount on what she should have paid.

Now, after a three-year battle, the case has ended up in the Civil Court for a judge to decide whether she will have to pay the GST component back.

Harris, who is based in Australia and appeared via video link earlier this week, enlisted the services of solicitors Toby Braun and Kevin Bond when she entered a settlement with her now ex-husband in 2019.

When the legal fees were written up at $52,000 that amount excluded GST, but an error made by the cost assessor meant the bill mistakenly said GST was included.

Harris, who told the court mistakes were made continuously throughout the case, feels she's more than paid for the work done by the lawyers.

"I honoured the law and paid the money promptly, I feel like I have overpaid."

Lawyer Julian Long, who represented Braun and Bond in the High Court at Auckland, said the error was not the solicitor's fault and argued that a slip rule - a High Court rule that allows for a judgment or order to be corrected by the court if a clerical mistake has been made - should apply.

"They invited the Lawyers Standards Committee to change the error and relied on the 'slip rule' which is designed to allow the appropriate committee to make a change if it does not express what was decided."

However, the committee said they couldn't apply the "slip rule" and declined the request.

"This is the most straightforward error of law here, it doesn't matter if you characterise something as a slip or not when you are confronted with an issue like this, the only proper outcome is to fix the error," Long said.

The Standards Committee took the matter back to the assessor who advised he did make an error.

Consent of both parties was required to fix the error and Harris resisted the review of money, opposing the solicitor's application to correct the error.

The ongoing battle means the lawyers have still not been paid the full sum.

Harris said her father, a lawyer based in Australia, had been communicating with the law firm.

"I never received any letter asking for my consent, my father was acting for me and the parties were communicating with him, they must have not known that he had passed away."

"My father was acting as my cost assessor too, he was the longest-serving lawyer in Australia and only retired three days before his death.

"He wanted to pass on in peace knowing I had paid my debts, which I did. I didn't expect to hear back from anyone again over this court case."

Justice Gordan said the issue lies with the error of GST, not the price in full.

A reserved decision is expected in the following months.

- Ellen Thomspon, Open Justice