Labour leader Andrew Little said clinical trials show Keytruda, a melanoma drug, was effective for at least one in three patients.

Patients wanting access to groundbreaking drugs that are still being tested would get Government funding if Labour gets into power.

Opposition leader Andrew Little said "by and large" he didn't have an issue with Pharmac, the national drug purchaser, but he was in favour of an early-access funding scheme, similar to the United Kingdom.

Health Minister Jonathan Coleman has ruled out providing early funding for new drugs where results appear to be promising, but there may not be sufficient data to justify full funding.

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Green Party health spokesman Kevin Hague criticised Little for his comments that politicians should get involved in decisions around what drugs Pharmac funds.

Hague said it was a "shame" that Little said Labour would fund Keytruda, a potentially life-saving treatment for melanoma sufferers.

"Political decision-making on medicines a very bad idea," he posted on Twitter.

But Little said Pharmac was already constrained by politics.

"Politics will decide what its budget is, politics will decide what the priority areas are they have to deliver - it's a system that's not free of political judgement," he said.

Labour plans to campaign on five or six principal issues but "whether or not this is one of them remains to be seen, but that doesn't stop us making the promise that we'll fund (Keytruda) regardless of what our primary campaign topics are".

Little said there was a strong case for early-access funding and he would look at changing that within the Pharmac model if Labour wins the election in 2017.

On Friday National MP Judith Collins hinted the Government might look to fund the "game-changer" treatment.

She said to "wait and watch this space" and that "every now and then something comes along and you have to say 'something has to give on this'".

Little said Key should "do the right thing" and provide Government funding to Pharmac for the drug.

But a spokeswoman for Coleman said on Sunday that nothing had changed and Pharmac was independent from Government.

Little, who has had his own battles with cancer, said he knew the "fear that comes from hearing that diagnosis".

"It must be devastating for melanoma sufferers to know there's a drug out there that could potentially save their lives, but that they can't get it in New Zealand."

While Keytruda is publicly funded in Australia and the UK, New Zealanders are forced to pay full-price, sometimes more than $300,000 for a full course of treatment, prompting Kiwi doctors to ask people if they had access to overseas treatment schemes.

Little said clinical trials showed Keytruda was effective for at least one in three patients.

"John Key needs to step in and do the right thing. It's not about picking winners and losers, it's about saving Kiwi lives," he said.