New Zealand's embassies have been involved in covert intelligence gathering work on behalf of the United States, a fresh batch of classified documents released by NSA whistleblower Edward Snowden appear to show.

The 2003 list of "Exceptionally Controlled Information" which is itself classified top secret gives a brief rundown of various NSA intelligence gathering operations and their security codes and status.

Under the heading "Stateroom" the document describes the security classification that "restricts access to information about personnel involved in covert collection, processing and report of SIGINT [signals intelligence] from diplomatic missions abroad".

"This activity is conducted by USA, AUS, CAN, GBR and NZL", the document states.

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Suspicions around New Zealand's involvement in signals intelligence gathering on behalf of the US National Security Agency were reinforced late last year when documents released by Mr Snowden revealed the existence of the Stateroom programme.

The documents showed the programme operated from sites at US embassies and consulates and from the diplomatic missions of intelligence partners in the so-called Five Eyes intelligence gathering and sharing network, including Australia, Britain and Canada.

It was reported at the time that activity linked to Stateroom saw Australian embassies in Jakarta, Bangkok, Hanoi, Beijing and Dili, and high commissions in Kuala Lumpur and Port Moresby operated surveillance collection facilities, in many cases with diplomats unaware of them.

The new documents leaked by Mr Snowden were published over the weekend on The Intercept website which is run partly by US journalist Glenn Greenwald.

Mr Greenwald promised to reveal details of New Zealand's involvement in spying on friendly countries and trade partners when he was in this country last month for Kim Dotcom's Moment of Truth.