Department of Prime Minister and Cabinet on Monday told Senate Estimates it had not taken any action on cabinet ministers and staff using online messaging platform WhatsApp, and then sought further legal advice about the risks involved.

"I undertook to go and seek further information from (the Australian Signals Directorate) and I've had a conversation," the prime minister's chief cyber security adviser, Alistair MacGibbon, said in Senate Estimates later on Monday.

"Encrypted over the top applications such as WhatsApp provide users with significant amounts of privacy," he said.

Mr MacGibbon had initially refused to answer whether he had sought security advice, instead telling Labor's Penny Wong to direct its questions to the Australian Signals Directorate.

The department said it began initial queries with ASD about the service over the weekend.

Earlier, Alistair MacGibbon confirmed he does use WhatsApp to communicate with Prime Minister Malcolm Turnbull.

“I have communicated with several people inside the (Prime Minister’s Office),” Mr MacGibbon said of the communications when asked in Senate Estimates.

“He has communicated with me on WhatsApp and nothing of all of a national security nature.”

Attorney-General George Brandis defended the use of the online service and said he too communicates with the prime minister as well as cabinet ministers.

“Never ever, ever, has there been an occasion when a sensitive or national security matter has been reduced to that form for obvious reasons, ” he said.

"Nothing of public sensitivity ... it is entirely unremarkable."

Senator Wong quizzed whether a specific cabinet member group existed on the messaging service but Attorney-General Brandis avoided the question.

"I undertook to go and seek further information from (the Australian Signals Directorate) and I've had a conversation."

“I don’t want to essay a view as to what the technical description of, um, a communication is,” Senator Brandis responded.

The Attorney-General said he would not be exporting his chats as a text file to release to Senate Estimates.

The prime minister’s department confirmed it had not taken any action over the revelations and had not issued updated security advice to staff members or parliamentarians using the service.

“There’s probably an app a day released and so we’d be chasing our tails to say ‘well you can use this app, can’t use that app,’’’ Deputy Secretary of National Security within Prime Minister and Cabinet Allan McKinnon said.

“Are you just hoping and praying that no information that is classified, sensitive or of national security character is being communicated by the government ministers or staff?” Senator Wong asked.

“It’s as sensible as me going to them and saying, ‘are you aware you shouldn’t be discussing classified information on the telephone?”’

The Opposition said the use of the app is akin to the government treating national security with contempt and asked whether the messages were subject to Freedom of Information requests.

Prime Minister Turnbull's cyber security adviser Alastair MacGibbon said the FOI Act does allow for the messages to be released, but wasn’t sure whether they could be retrieved.