"A business is subject to the 12-hour time frame after it becomes aware of a critical incident, such as ransomware or unauthorised access to an asset."
I think that's a fair timeline especially after they become aware of the incident. I didn't read the SOCI Act but I'm assuming the Government has additional support and resources that can be utilized after reporting the incident?
This is one of those situations where as soon as you receive the letter as being identified as "critical infrastructure", locate the reporting website/phone number and update it in the Incident Response plan ASAP.