"In the choice between a cock-up and a conspiracy, this one was absolutely in the cock-up end," he told journalists in Auckland on Friday.
However, the ombudsman has re-opened an investigation into the handling of a revelatory email, a probe likely to produce more grief for Mr Hipkins' under-pressure government.
The prime minister has endured his toughest political week of his two-month-old leadership, all thanks to the axed Mr Nash.
On Tuesday, he sacked his economic development minister from cabinet when it was reported Mr Nash had revealed confidential cabinet discussions to two businessmen and political donors.
In 2020, Mr Nash, then small business minister, sent an incriminating email to the two men explaining a decision on commercial rent relief during COVID-19 lockdowns.
The correspondence breaches NZs cabinet manual, and was labelled by Mr Hipkins as "inexcusable".
The scandal deepened on Wednesday when it became apparent the office of the previous prime minister Jacinda Ardern was aware of that email due to an Official Information Act (OIA) request made the following year.
NZ's OIA laws are similar to Australia's Freedom of Information laws, which allow Kiwis to request many government documents and communications under the public interest.
On Thursday, it emerged that OIA request was declined - most likely wrongly - on the basis it did not pertain to Mr Nash's ministerial duties.
Mr Nash's and Ms Ardern's offices corresponded multiple times about the OIA request, giving rise to the suggestion of a cover-up.
In Question Time, deputy opposition leader Nicola Willis railed against the decision not to disclose the document, calling it a "conspiracy".
"Isn't it actually the case that the prime minister's office saw the email and, instead of doing the right thing, actively chose to cover it up," she said.
Mr Hipkins, who was not in the house on Thursday as conventional, responded from the city of sails a day later.
"Clearly in this case, a mistake was made in the prime minister's office," he said.
"Ultimately the accountability for that, the responsibility for that, still sits with the minister to whom the request was addressed.
"That accountability sits with Stuart Nash and he's being held accountable for that."
Later on Friday, Chief Ombudsman Peter Boshier said he would take another look at the case, which was discontinued last year.
"I will consider whether or not the information in question was held in his capacity as a minister, which is subject to the OIA, or as an electorate MP that is not subject to the Act, and whether there was a basis for withholding the information," he said.
Ms Willis said the decision to withhold the email could be criminal.
"What we have here is a government which only remembers its ethics when it gets found out," she said.
"It seems that the prime minister's office conspired with Stuart Nash's office to keep the email secret."
Mr Nash's sacking comes after a string of scandals, including him calling up the police commissioner to request the appeal of a sentence.
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