Trans Pacific Partnership negotiations have been extended by another day.

Trade Minister Tim Groser is in Atlanta for the talks but Prime Minister John Key is heading home from New York today.

There's disagreement over how the US opens its market to auto parts from Japanese car makers, and how much Canada is willing to open its market to dairy products, such as cheese, from Australia and New Zealand.

The US is also pushing to establish longer patent protections for drugs made using living organisms.

The deal would create the world's largest free trade zone.