Leaked documents from the Trans-Pacific Partnership Agreement (TPP) show giant drug companies are trying to control the price of their products and deprive New Zealanders of access to cheaper medicines, opponents say.

Three secret US-proposed texts from the negotiations in Lima, Peru, were leaked and posted, along with critiques, on the Citizen Trade Campaign website yesterday.

The TPP is a regional trade agreement 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 TPP opponent Jane Kelsey, a University of Auckland law professor.

Pharmac is the government agency that buys drugs for New Zealand, and can negotiate with manufacturers to get better prices.

Professor Kelsey said the secret TPP negotiations gave foreign corporations enormous leverage over democratic processes and threatened the country's health care system.

American legal expert Professor Sean Flynn said the US proposal would regulate public health policy and had no place in a trade negotiation.