The acceptance of Keytruda's effectiveness after Pharmac's budget was boosted undermines the medicines funding agency's credibility, Labour says.
A campaign by Labour, some doctors and patients for an advanced melanoma drug to be funded - with Keytruda most often mentioned - ran earlier this year.
Pharmac and Health Minister Dr Jonathan Coleman cited a lack of evidence about Keytruda's efficacy, and said evidence presented by maker Merck Sharp & Dohme had not been peer-reviewed.
After a $39 million boost as part of last month's Budget was confirmed in May, Pharmac proposed to fund rival melanoma drug Opdivo.
And this week it published a new provisional agreement to fund Keytruda.
From September 1, patients would be able to switch melanoma drugs within 12 weeks of starting if their first one was intolerable and their disease had not progressed.
Labour's health spokeswoman Annette King said some patients had died waiting for Keytruda, and others had taken out mortgages or turned to friends and family, or even the Givealittle website, to fund it themselves.
She was delighted patients would soon be able to access the drug.
"But what is inexplicable is, having told the public of New Zealand - both the minister and Pharmac - that the efficacy of Keytruda wasn't as good as Opdivo, that it hadn't been peer-reviewed and they weren't prepared to fund it, weeks later [Pharmac] are now consulting to fund it...and absolutely nothing has changed from when they made those comments.
"And I believe it is a political move...I believe [the Government] didn't direct Pharmac to fund Opdivo, they just made it clear they were not happy to have Keytruda funded."
Sarah Fitt, Pharmac's director of operations, said the agency had remained open-minded to funding Keytruda, and had worked with Merck Sharp & Dohme to reach a favourable commercial agreement.
"As this is a new treatment area, Pharmac continues to closely monitor developments in evidence and treatments", says Sarah Fitt, Director of Operations.
"During recent consultation on the funding of Opdivo, Pharmac received a number of submissions from medical oncologists that the overall evidence for melanoma treatments supported treating Opdivo and Keytruda as interchangeable treatments."
Dr Coleman said that Pharmac was best placed to comment on evidence for drug proposals, but its "model for increasing subsidised medicines and treatments for New Zealanders is world class".