Immigration New Zealand is facing an independent financial audit after revelations that its bungled technology upgrade cost at least $6 million more than thought.

Last month, Immigration Minister Erica Stanford accused officials of deliberately withholding information about the mess and using “creative accounting” to avoid scrutiny.

At the time, she said more than $30m had been wasted with nothing to show for it.

On Wednesday, Stanford said her staff had since discovered an error in information provided by the Ministry of Business, Innovation and Employment.

In response to questions, the ministry’s chief executive Nic Blakeley had advised her that another $6m in “associated project costs” had been identified.

“The chief executive informed my office today that he cannot be confident this further $6 million represents the full extent of expenditure… and he has made the decision to commission an independent financial audit,” Stanford said.

That raised “significant questions” about the ministry’s financial controls, so she had referred the matter to the public service commissioner and the Michael Heron inquiry that was currently underway, she said.

The commissioner would also consider whether the inquiry’s scope needed to be widened.