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Some of New Zealand's biggest stars are backing a new Greenpeace climate change campaign leading up to a major world summit in December.

The "Sign On" campaign was launched yesterday at a celebrity-studded barbecue at international star Lucy Lawless' Mission Bay mansion.

The campaign aims to persuade John Key to sign on to a target of cutting greenhouse gas emissions by 40 per cent by 2020.

"Sign On" ambassadors include actors Rhys Darby, Robyn Malcolm, Cliff Curtis and Keisha Castle Hughes, chef Peter Gordon, and Stephen Tindall.

New Zealand's current goal is 50 per cent of 1990 levels by 2050. This compares to the United States' goal of reverting to 1990 levels by 2020.

Lawless said in a media release: "I'm signing on because there's no Planet B. There's nowhere else for our kids and grandchildren to go. Our Government needs to sign up to a 40 by 2020 target."