Who is a PCBU
I have spent a lot of time trying to explain this to senior managers and owners as it is quite confusing. It is the phrase 'Person' in PCBU that misleads folks. PCBU is the company or organisation as a legal entity - the company has an obligation to look after anyone who works for it, does work for it (contractors) or could be affected by work it carries out. It does this by using systems and procedures such as risk analysis, accident reporting and so. Business owners/directors etc. (Officers) act as kind of a backstop for this, they are there to make sure the company (PCBU) is doing what its supposed to to keep those folks safe. They can do this by making sure that those procedures are up to date, emergency drills have been carried out, accidents are being reported and investigated to mention just a few of their duties. This is traditionally done at BoD meetings, but they should be taking a more hands role, getting out into the workplace to talk to workers and getting an understanding of what their people face everyday.
So, to answer your question who in the company is accountable? Nobody is accountable in the sense that if the company was prosecuted it is the company that would be fined and sanctioned. Think about which bank account the fine would come out of! The directors/owners would not personally suffer financially but the company would. This is different to the Officer being prosecuted - that is a personal fine/sanction.
In another, more practical sense everyone is responsible to make sure they follow process and instruction, failure to do this means the company may have to explain why those processes weren't robust enough to prevent someone getting hurt.
I apologise if this is overly simplistic, if you need help please feel free to shout out on alandotboswellatactiverefrigdotcodotnz