An Australian intelligence agency has been collecting thousands of private email and chat contact lists in its attempts to assist the NSA, new leaks about the US spy agency’s snooping capabilities have revealed.

The latest information leaked by the whistleblower Edward Snowden to the Washington Post has revealed extensive collecting of private contact lists and address books from email and instant messaging accounts, a program that was previously undisclosed.

The leaked document – a PowerPoint presentation on “content acquisition optimisation” – explains how the NSA can address the challenge of having too much data from its interceptions and those from the “five-eyes” partner agencies in Australia, Britain, Canada, and New Zealand.

One slide states that “the NSA collects, on a representative day, ~ 500,000 buddy lists and inboxes”.

More than 250m address books are gathered every year, collected “on the fly” as they are transmitted.

On a single day used as an example, the address book haul included 444,743 from Yahoo, 105,068 from Hotmail, 82,857 from Facebook, 33,697 from Gmail and 22,881 from other providers. A third of them came from an Australian Defence Signals Directorate (DSD) search.

A case study of the email account of an Iranian target was mined by the DSD on behalf of the NSA.

The Washington Post reported that these lists could provide greater personal detail than phone records, and often included some message content. But because of the vast network created by address book contacts it could also create false representations of associations between people.

As with previous revelations about the NSA’s surveillance programs, authorities gave some justification that their searches concentrated on overseas targets. An anonymous official told the newspaper that when information passes through the data collection program, “the assumption is you’re not a US person”.

A spokesman for the Office of the Director of National Intelligence, which oversees the NSA, told the Washington Post that the NSA “is focused on discovering and developing intelligence about valid foreign intelligence targets like terrorists, human traffickers and drug smugglers. We are not interested in personal information about ordinary Americans.”

A spokeswoman for the attorney general told Fairfax media that all Australian agency interceptions were in accordance with Australian law. ''Our intelligence agencies operate under a strong legal and oversight framework,” she said.