Labour health spokeswoman Annette King said an extension to drug-patents could add $1 billion to the national health bill.

A five-year extension to drug patents could cost taxpayers an extra $1 billion, says Labour.

Talks on the 12-nation Trans Pacific Partnership deal are in their final stages and Prime Minister John Key has already said the deal would likely see patent protection for some drugs extended beyond the current period - effectively about five years from introduction to the market - pushing up the cost.

The Government has downplayed the extent to which an extension to patents would affect New Zealanders.

Key has said patients would not pay more than the current $5 fee for subsidised prescriptions. Any extra cost to the medicines bill as a result of the free trade deal would be absorbed by the Government.

But Labour health spokeswoman Annette King said Key was playing with words.

Figures from the national drug-purchasing agency, Pharmac, showed it spent $800m on medicines in 2014 that would have cost $2.2b without the deals it was able to negotiate when medicines came off patent, King said.

Pharmac reported permanent savings of $40m to $50m a year through those deals.

"The effect of lengthening patents for five years on medicines from TPP countries, which produce half of the world's pharmaceuticals, would be to halve the flow of medicines coming off patent for those five years," said King.

Cumulatively, that could equate to $1b in lost savings over 10 years.

"Of course it will cost New Zealanders in two ways," said King.

"One is cost in terms of taxpayers. It's the taxpayers who will have to pick up the additional costs for pharmaceuticals. And the second is this delay in us getting access to generic drugs for the new modern drugs that are coming out.

"That is a delay to sick patients who can't get access to medicine at an affordable cost."

Senior medical specialist Dr Erik Monasterio said the impact of extended patents would be significant.

Monasterio has been a vocal critic against the secrecy surrounding the 12-nation free trade deal, which is in the final stages of being hashed out.

He dismissed claims from Health Minister Jonathan Coleman in Parliament that changes to patents under the deal would be small and the Government would fund them.

"Without sounding unscientific, that's total rubbish.

"The costs are not small. I'm disappointed and surprised that Government leaders can talk about figures that are likely to affect the population in such vague terms."

He cited the purchase of the generic version of one anti-psychotic medication (Olanzapine), in 2010, which led to $30m in savings alone.

"It's all very well for the Government to say it's going to fund [extended patents], but if that is the case how can the Government hold that promise at the same time it's putting enormous pressures on district health boards to curb their budgets?"

Pharmac operated on a capped budget of about $800m. Monasterio said that as extended patents drove up the cost burden, Pharmac would be left with fewer medications it could afford to purchase.

Key, who was in Singapore, was unable to comment, his office referring questions to Trade Minister Tim Groser's office.

Groser said the Government had been "repeatedly clear" it would not negotiate on the "fundamentals of Pharmac".

"This includes the Pharmac model which has been so successful over the years. This position will not change."

Analysis before the deal was signed was "purely speculative" Groser said.

"The Government is taking a careful approach to pharmaceutical patent issues in TPP to ensure any agreed outcome does not impact medicines access or costs to New Zealanders."