The tribunal suppressed the names of innocent parties implicated in her actions, which were cited as being unbecoming to an accountant.
Williams, a business services manager, was accused of breaching the institute's rules and code of ethics, in particular fundamental principles of integrity and quality performance.
Tribunal members were told invoices were sent to an important client which said financial statements had been prepared when that was not the case.
When asked why the statements and tax returns did not appear to have been completed, she said the client had two IRD numbers and the tax returns were filed using an incorrect IRD number - neither of which was the case.
She was difficult to get hold of when senior managers wanted to discuss the omissions and there appeared to be little on the files.
Williams said the paperwork they wanted to check was at home and, later, that she could not find it. Eventually she resigned.
Managers discovered she had logged on to the company's current computer system to file outstanding returns last year but later denied any involvement in that. No supporting evidence was supplied to justify figures in the tax returns, which were not prepared using Hayes Knight software.
In some cases, GST returns were not filed by due dates, those that were had no supporting data, and some figures were fudged.
A director whose name was suppressed said some work did not seem to have been done at all.
"I consider this to be false invoicing because there is no evidence of preparation."
Williams deliberately misled directors with plausible excuses, promises and by hiding evidence, he said.
It had cost Hayes Knight about $150,000 in time to unscramble the mess. In addition, the firm had refunded clients for fees and IRD charges.
The tribunal found Williams guilty, struck her off and ordered costs of $9155.
Outside the hearing, another director said there appeared to be no logical motive for Williams' actions.
"She made no money out of the situation. We trusted her. The clients trusted her."
Hayes Knight did all it could to "make it right" for the clients affected after Williams resigned. One of the clients who had come with her when she joined the firm had now moved to another accountancy practice.