The pharmaceutical industry has given millions of dollars to 28 United States senators lobbying for an overhaul of Pharmac in trade talks with New Zealand.

The revelation comes as new leaks show American negotiators are set to take a hard line on the New Zealand Government's drug-buying agency in TransPacific Partnership trade talks.

In May, the senators wrote a letter to US president Barack Obama demanding changes to Pharmac who have received $6.5 million in donations from the pharmaceutical industry over the past five years.

The senators want Pharmac, which has a set budget and tries to find the best value drugs on the market, to pay more for medicines that come from American pharmaceutical companies.

The figures are on the website of the Washington-based Centre for Responsible Politics and have been published as details emerge of America's official position on Pharmac in the TPP talks.

The Washington College of Law's Sean Flynn says if New Zealand agrees to the demands, it will fundamentally change the way Pharmac bargains with drugs companies, forcing it to pay more for medicines.