Opposition leader Todd Muller says David Clark's resignation as Health Minister "was too little, too late".
Clark announced on Thursday morning the "time is right" for someone else to fill the role and said he'd become a "distraction" in New Zealand's COVID-19 response.
Muller said Clark hadn't delivered in his role as Health Minister and there was "no question he had to go".
"National has been calling for his resignation for months following successive failures, including breaking lockdown rules and, most recently, having two women leaving quarantine who weren't tested for COVID-19, under his watch," he said.
"Clark did not deliver to the standards that we expected, neither through the lockdown or the border management in recent weeks, which has been utterly shambolic."
He said Prime Minister Jacinda Ardern must take responsibility after she "failed" to fire Clark in April when he offered his resignation.
He was caught in April driving over two kilometres in order to visit a mountain bike track while the country was in alert level 4. Four days later, he also admitted he'd driven more than 20 kilometres to visit a beach with his family. Later in April, Newshub revealed he'd moved house during lockdown - another thing Kiwis were told they couldn't do.
"She declined Clark's offer to resign when he decided to break his own lockdown rule not once, not twice, but three times in April," Muller said.
"The question has to be asked; why did she repeatedly fail to fire him?"
Minister of Education Chris Hipkins is taking over the health portfolio until the September election, something National's deputy leader Nikki Kaye says the party is "very concerned" about.
"Being Minister of Health in a global pandemic should be a full-time job. So the idea that Chris Hipkins can do both Minister of Health, Education and State Services in the middle of a global pandemic we don't think is a good idea," she said.
"The other thing we'd say is it shows a lack of depth in Labour's batting order, that they have to give the Minister of Health to Chris Hipkins."
Ardern said she has "full confidence" in Hipkins overseeing the health portfolio since he has experience with an operational ministry of significant scale.
"Our COVID response is an all-of-Government response. Making sure that you have a good understanding and awareness of how to draw in the public sector as effectively as possible is one of the strengths that Minister Hipkins has."
She lauded Clark for putting the interests of his team above his own.
"I want to thank him for his service. He delivered a lot in his three years as Health Minister and there are many things that he can be proud of."
Clark said he will still stand as an MP in his local electorate Dunedin North in the upcoming election.