It's only in the last few months that Pharmac obtained enough evidence to justify funding the melanoma drug Keytruda, it says.
The drug-buying agency has made two drugs, Keytruda and Opdivo, available to advanced melanoma patients from today.
The agency has been criticised for not funding the drugs earlier.
But Pharmac's director of operations, Sarah Fitt, told Checkpoint with John Campbell the drugs were very expensive and it needed to wait for further clinical research to justify the purchase.
The agency had been actively seeking out evidence and talking to clinicians for the last year, Ms Fitt said.
New trial research released in the last few months got the funding decision over the line, she said.
Watch interviews with Ms Fitt, Cancer Society head Dr Chris Jackson and an advanced melanoma patient taking Keytruda, Barry:
