Regardless, it is Key as chairman who is responsible for the issues emerging under his watch.
The highly unusual nature of this departure puts more pressure on the former Prime Minister.
It is the second serious issue for the bank this year.
Last month, the Reserve Bank of New Zealand revoked ANZ Bank New Zealand's accreditation to model its own operational risk capital requirement due to a "persistent failure" to properly calculate risk.
It was ordered to increase the capital it holds as a safety net to absorb possible losses, by 60 per cent to $760 million.
That prompted former BNZ chairman Kerry McDonald to write to Reserve Bank governor Adrian Orr, saying he was "amazed" at the limited penalty imposed on the bank.
He called for ANZ chief executive David Hisco and Key to resign.
ANZ said it regretted the error, but also that the sum involved was a small portion of its total capital and that it had "no impact on customers or the operation of the bank".
Soon after Hisco departed on sick leave. Key did not comment publicly about the breach.
Today he described them "as very separate issues".
He also attributed blame for the breach to an error by a junior staffer.
Both issues will be the subject of reviews.
Key also sits on the board of the ANZ's Australian parent company - something big bank critics like fund manager Sam Stubbs has described as a conflict of interest.
Stubbs, a former banker and chief executive of Tower Insurance, described the capital breach as "very concerning behaviour by our biggest and most profitable bank".
"This is as strong a censure of a bank as I have ever seen in New Zealand."
Hisco's departure today comes with a game of high stakes at play in the New Zealand banking sector.
The Reserve Bank is proposing to increase the amount of capital all the banks must hold in order to ensure they are safer and less vulnerable to financial shocks.
But it's a move that could cost shareholders billions in profits.
Unsurprisingly the banks have pushed back, arguing the Reserve Bank is overcautious and the costs will be borne by New Zealand borrowers.
Publicly the language has remained formal, but behind-the-scenes tensions are understood to be running very high.
Meanwhile, the Australian parent companies are already under pressure in their own market.
The Hayne Royal Commission into banking conduct found serious deficiencies in the way the banks were operating in Australia.
There are now renewed calls for a royal commission here.
Even as unrelated events, these add up to a bad look for ANZ at a time when it could ill-afford another scandal.
