Leaked documents from the Trans-Pacific Partnership Agreement show giant drug companies are trying to control prices and deprive New Zealanders of cheaper medicines, opponents say.

Three United States-proposed texts from the Trans-Pacific negotiations were leaked and posted on the Citizen Trade Campaign website yesterday.

The regional trade agreement is between New Zealand, the US, Australia, Brunei, Chile, Malaysia, Peru, Singapore and Vietnam.

The combined effect of the three leaked texts gave big pharmaceutical companies "a platform to wage a war of harassment against Pharmac", said agreement opponent Jane Kelsey, an Auckland University law professor.

Pharmac is the government drug-buying agency and negotiates for better prices.

"The US proposals would allow drug companies to challenge every Pharmac decision as not appropriately recognising the `value' of patents – a dangerous and undefined standard," Prof Kelsey said.

"Adopting this standard would open floodgates of litigation against Pharmac and ultimately raise medicine prices and ration access."

She said the secret negotiations gave foreign corporations enormous leverage over democratic processes and threatened the viability of the country's healthcare system.