The Australian Signals Directorate (ASD) has released detailed technical guidance instructing critical infrastructure operators on how to fully separate operational technology and "vital enabling" systems from the outside world when crisis or conflict strikes.

Its revised CI Fortify-Advice for isolating vital systems sets a three-month target on how long an operator should be able to run its essential services in a state of complete disconnection from other systems and from the internet. The new guidance follows a 12-month consultation with the industry.

The revision comes against a backdrop of a continually challenging threat environment, in which state-sponsored espionage and ordinary cybercrime are increasingly blurring together. "Sometimes cyber-attacks are blending both criminal and geopolitical objectives," ASD's assistant director-general for cyber uplift Heidi Hutchison told iTnews.

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Full isolation is not the initial response, however. "It's actually probably the last measure of resilience," Hutchison said. Instead, ASD suggests a graduated plan that steadily reduces exposure to external networks before an operator needs to fully "pull up the drawbridge".