Always? No. Sometimes? Yes.
The Corporations Act 2001 (Cth) imposes duties on officers and employees. Officer includes, as well as directors and secretaries, some other people who manage the corporation or its property (such as the CEO). Employees includes the CEO but not non-executive directors.
ss180-183 impose duties of care and diligence, good faith, use of position and use of information, the breach of which are civil offences. s184 defines when the breach of these duties rises to the level of a criminal offence. In particular:
Good faith--directors and other officers
(1) A director or other officer of a corporation commits an offence if they:
(a) are reckless; or
(b) are dishonest;
and fail to exercise their powers and discharge their duties:
(c) in good faith in the best interests of the corporation; or
(d) for a proper purpose.
You start with "If a CEO was largely or completely unaware of the situation" which begs the question why was the CEO unaware?
The duty under s180 is pretty clear:
(1) A director or other officer of a corporation must exercise their powers and discharge their duties with the degree of care and diligence that a reasonable person would exercise if they:
(a) were a director or officer of a corporation in the corporation's circumstances; and
(b) occupied the office held by, and had the same responsibilities within the corporation as, the director or officer.
Prima facie a CEO has a duty to be aware of what the company is doing and to have introduced appropriate policies, procedures, practices and auditing to detect any criminal activities of the company in a timely manner.
Obviously, they can't be aware of everything the company is doing (unless they are the only employee of the company) but their duty, along with the other officers, is to ensure "with the degree of care and diligence that a reasonable person would exercise" that the company doesn't break the law and, if it does, to detect and deal with that.
In the case of Enron, where the officers knew the company was breaking the law and actively participated in that then they were acting dishonestly and their actions would have been criminal under s184 alongside any other crimes they personally committed.
In the same vein, a CEO who took no action at all to address risks of criminal activity, such as a financial company with no auditing, could be verging on reckless and thus cross over into criminality.
Work Health and Safety Law
new-south-wales although WHS law is largely consistent across Australian jurisdictions.
s19 or the Work Health and Safety Act 2011 (NSW) imposes a non-transferable primary duty (other sections impose specific duties subordinate to the primary duty) on "Persons Controlling a Business or Undertaking" (PCBU) which includes the CEO:
(1) A person conducting a business or undertaking must ensure, so far as is reasonably practicable, the health and safety of--
(a) workers engaged, or caused to be engaged by the person, and
(b) workers whose activities in carrying out work are influenced or directed by the person,
while the workers are at work in the business or undertaking.
(2) A person conducting a business or undertaking must ensure, so far as is reasonably practicable, that the health and safety of other persons is not put at risk from work carried out as part of the conduct of the business or undertaking.
There are 3 categories of criminal offence for breaching that (or any other) duty.