A high-profile American whistleblower and a privacy researcher have been unexpectedly dropped from addressing a Government-backed cybersecurity event underway in Melbourne.

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Thomas Drake and Dr Suelette Dreyfus of the University of Melbourne were both told their talks were "incongruent" with CyberCon, despite being invited to speak months earlier.

Mr Drake's presentation was to address national security and surveillance, while Dr Dreyfus planned to explore the use of safe digital drop boxes for anti-corruption whistleblowing.

A former National Security Agency (NSA) official, Mr Drake brought to light concerns about internal agency waste and surveillance in the mid-2000s. He said he received notice about the CyberCon decision shortly before boarding a plane to Australia.

"Apparently, a partner of [Australian Information Security Association] did not want to have me, as a whistleblowing voice, on stage in front of the delegates," he said.

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Dr Dreyfus said the conference organiser, the Australian Information Security Association (AISA), also told her a conference partner made the decision.

The conference is supported by the Government's Australian Cyber Security Centre (ACSC), among other sponsors, which coordinates Australia's cybersecurity response.

It is also part of the Australian Signals Directorate security agency — this country's counterpart to the NSA.

AISA's deputy chair Alex Woerndle confirmed that Mr Drake and Dr Dreyfus were replaced in the conference program only recently.

He said detailed questions about the incident were best answered by the ACSC.

"AISA supports and encourages diversity of views however it's important to note we work with a number [of] partners, including Government, and as such need to manage a variety of views to deliver an event catered for all our stakeholders," he said.

The NSA whistleblower Thomas Drake was surprised to be dropped from the conference program. ( ABC News: Thomas Drake )

The Shadow Assistant Minister for Cybersecurity Tim Watts called on the Government to explain "why it wants to silence speakers on whistleblowing".

"If the Government has legitimate concerns over these individuals, then they should be upfront and explain themselves," he said.

The ACSC did not respond to request for comment by deadline.

Former speaker: Cancellation is 'hypocritical and ironic'

Dr Dreyfus said she had no warning about the cancellation of her speaker slot, and was surprised by the development, especially in light of the ongoing debate in Australia over whistleblowers and press freedom.

"There's now a culture of fear about speaking up," she said.

"Nothing highlights this quite so much as disinviting speakers who have been confirmed."

Both Dr Dreyfus and Mr Drake said they were notified of the decision just over one week ago, but their attempts to discover more detail about reasons for the cancellation were rebuffed.

Mr Drake said this was the first time he had been stopped from speaking after being invited to a conference.

In his view, whistleblowers are "the canary in the coal mine" of democracy.

"I find it hypocritical and ironic that I would be silenced at this conference," he added.

Melbourne University academic Suelette Dreyfus was set to talk about whistleblowing technology at CyberCon. ( ABC News: Suelette Dreyfus )

Dr Dreyfus said removal of Mr Drake's talk from the program was a missed opportunity.

"Silencing him when he's travelled 10,000 miles to not speak on a podium is Australia's loss and it's the cybersecurity [community's] loss," she said.

According to Dr Dreyfus, this incident is damaging to Australia's cyber security readiness, which requires diverse points of view.

"That is how you are strong. And if you silence experts, if you silence expert voices, your team is weaker," she said.

Both individuals are in Australia, and still able to attend the conference and speak with other delegates.

Professor Mark Cassidy, dean of Melbourne University School of Engineering, said they were "disappointed" to learn Dr Dreyfus was disinvited from presenting at the conference.

"Now is the time for important and robust discussions about maintaining data security and protecting Australians," he said.

"We welcome debate and will always support academic freedom."

Mr Drake was charged for his actions at the NSA under the United States Espionage Act, but those charges were later dropped.