But the archives' director-general David Fricker doubts his agency has received any records of WhatsApp conversations from the public service for storage, despite the "absolute certainty" that ministers and their advisers communicate with bureaucrats through the service.

Mr Fricker delivered his frank assessment of record keeping for encrypted messaging among government officials during a parliamentary hearing into the archives' storage of digital material on Wednesday.

It means that crucial conversations about government and policy are likely not being recorded for later scrutiny - and that deliberations and public service advice leading to key decisions will stay off the historical record.

Public servants are required to keep records of official business they conduct on any platform, however the National Archives chief says his agency has no power to compel officials to record their encrypted conversations.

Under the archives act public servants cannot destroy Commonwealth records. But the legislation does not cover encrypted messages - which are property of the messaging services - if a separate record has not been made of them such as a screenshot.

Mr Fricker urged public servants conducting government business on services including WhatsApp, Signal and Telegram to file the conversations into their agencies' records.

He said it was "the clear message" that federal bureaucrats should either record their encrypted conversations about government business, or otherwise not use the services. However it remained to be seen how much that message was getting through to public servants, the archives director said.

Under questioning from Labor MP and public accounts committee deputy chair Julian Hill, Mr Fricker said it was an "absolute certainty" that government officials were conducting government business over WhatsApp.

However the director-general said he would be surprised if the National Archives had received any records of encrypted conversations.

Disappearing messages - a feature of messaging service Signal - were the "digital equivalent of a post-it note" or a phone call, he said, and public servants had a professional responsibility to maintain a record of those communications.

Mr Fricker said outdated archives laws created a "grey area" on keeping records of conversations conducted on platforms not owned by the Commonwealth, such as WhatsApp.

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The legislation should be updated to include a "more 21^st century definition" of Commonwealth records that included encrypted messages, he said.

"When you create a record, it has to meet those standards such that it is known, it is incorporated into an improved government information management system, be that third party or government-owned," Mr Fricker said.

"It is a pressing issue for us. The increasing use of third party, non-government, non-Australian platforms for the Commonwealth government's business does present a challenge.

"Are we keeping minimum evidence of official conduct that is created when we are using those non-government platforms?"

The National Archives has advised the Attorney-General's office, and is discussing changes to the archives act. A new definition covering Commonwealth records was among the priorities for change, Mr Fricker said.

The national audit office, one of the main federal integrity watchdogs, told MPs its job holding agencies to account would be easier if public servants kept records of their conversations on encrypted services.