Labour's Grant Robertson has argued that should be enough for Mr Key to consider Mr Banks had wrongly assured him he had complied with the law - the test Mr Key had set.
Mr Key said he now knew police believed the return was false, but that was not enough for him to move against his minister because they had made it clear they could not prove Mr Banks knew that when he signed.
Knowingly filing a false return is a serious offence with a punishment of up to two years imprisonment, whereas filing a false return without realising it was a minor offence and the one police believed they could have laid charges on but for the time limit.
Watching from the gallery, Mr Dotcom laughed whenever Mr Key said he believed he could still rely on Mr Banks' word and was clearly unimpressed afterwards.
Later outside Parliament, Mr Dotcom said Mr Key had a "fragile majority" and had to make a choice between upholding high ethical standards or staying in power.
"In my personal opinion I think if my leadership would not uphold high ethical standards, I would worry what else is lingering in the dark that I don't know about. As a voter, I would certainly consider who to give my vote to at the next election."
Dotcom was more generous about Mr Banks, saying although he did not believe Mr Banks should still be a minister, he had gone through a lot and "it's time to move on".
Asked why he had gone public about the donation, Mr Dotcom said he had been hurt by the way Mr Banks had publicly distanced himself from Mr Dotcom after his arrest and had refused to help him while in prison. "That's not what friends do."
Mr Dotcom is in Wellington for a Court of Appeal hearing today.