Good grief, it just keeps getting worse. Thanks for that info and all your other posts, @prh47bridge.
I worked as an auditor for a big chartered accountancy firm in the 1980s. I haven't worked in that field since then, but would be surprised if anything has changed in this one respect - auditors of large companies and other organisations check very few transactions indeed. They rely on the design of the accounting system. A good system, well run and adequately staffed, should make it difficult to get something wrong and those errors that do get through should be picked up and put right quickly. Fraud and theft should be very difficult to commit. Any organisation big enough to have its own internal auditors would normally be expected to need a lot less work from the external auditors.
The trouble is that many organisations are not well run and their accounting systems are not well designed and operated. The Post Office seems to be an outstanding example of this.
Also, auditors are not truly independent and that compromises their ability to tell the truth. In theory they are working for and reporting to whoever owns the business, not the directors, but they are appointed and re-appointed by the directors, who are responsible for preparing the accounts and therefore for any errors. If the auditors criticise the directors they could lose the audit, and audit fees are enormous.
Very rarely - in the case of a scandal as big as Mirror Group Newspapers pension scheme being robbed by Robert Maxwell without the auditors raising the alarm, for example - a big firm faces some sort of disciplinary process, strung out for years, ending with a fine for the firm which is a drop in the ocean of their massive profits. Then they just carry on as before.
I believe the external auditor for the Post Office for many years was Ernst & Young. PWC have the job now. Will E&Y face any sort of legal action or did they manage to cover themselves? There's usually some let out.