New documents reveal New Zealand spies and America's top government hackers cooked up a plan to spy on China, by cracking into a data link between Chinese government buildings in Auckland.

They're the latest revelations from Edward Snowden's leaked documents, and have been described as the most explosive yet.

The project appears to have been aimed at tapping data flowing between the Chinese consulate and its passport office in Great South Road, and using the link to access China's computer systems.

It's not known if the plan went ahead because, as sign-off was being sought, whistleblower Snowden walked out of his job with a trove of secrets.

A Chinese Embassy spokesman says China is concerned about the relevant report.

He says China attaches great importance to the cyber security issue, and will firmly safeguard its security interests.