Australia's defence minister, David Johnston, says the “5-Eyes” intelligence partners have achieved far too much with their co-operative surveillance programs to take a backward step now.

In a recording obtained by the West Australian newspaper, Johnston warns an audience of defence industry representatives to expect more damaging intelligence leaks courtesy of the US National Security Agency contractor-turned-whistleblower, Edward Snowden.

“We must assume the worst,” Johnston said. "Suffice to say it's an area that I can't get into in great detail, but I simply say assume the worst.

“We are watching with great acuity what is happening in the space.”

His remarks follow the latest revelation by Guardian Australia on Monday that Australia's surveillance agency offered in 2008 to share information collected about ordinary Australian citizens with its major intelligence partners.

According to a working draft memo recording the discussions in 2008, the Australian intelligence agency, then known as the Defence Signals Directorate, indicated it could share bulk material without some of the privacy restraints imposed by other countries, such as Canada.

The conversation referred to metadata – the information we all generate whenever we use technology, from the date and time of a phone call to the location from which an email is sent. It is unclear what if any decisions or specific actions followed the 2008 meeting at Britain’s GCHQ, at its headquarters in Cheltenham, on 22-23 April, 2008.

Johnston is the minister responsible for the Australian Signals Directorate.

He told the defence forum the 5-Eyes partners would press on with intelligence activities despite the disruptions created by the Snowden disclosures.

In comments apparently confirming co-operative surveillance activities, Johnston said the group had “invested far too much in this space to allow the event [the leaks] … to even contemplate a backward step”.

The group to which Johnston refers is Australia and its major intelligence-sharing partners, the US, Britain, Canada and New Zealand.

The revelations have raised concerns about the adequacy of privacy protections in an era of widespread digital surveillance and the Greens have called for a full parliamentary inquiry to ensure Australia’s intelligence agencies are not overstepping their legal mandates.

The prime minister rebuffed that idea on Monday.

Tony Abbott argued there was no evidence of overreach and argued the legal framework on intelligence gathering struck the right balance between protecting national security and protecting people’s privacy.

Abbott, like Johnston, signalled that he expected Australia’s intelligence services to remain vigilant and on task.

“Australia will act to protect our national interest and to protect our citizens,” Abbott told parliament on Monday.

“We always have under governments of both persuasions and, as far as I’m concerned, we always will – we will act to protect our national security.”