There is an interesting thread in which some of these issues are addressed:
A full systems audit is typically the largest and most extensive of audits. Registration audits are for all intents and purposes systems audits where all systems are sampled. Depending upon a company’s size, it may encompass 5 or more auditors over a week or more. Typical companies in the 100 to 250 range are about 5 to 8 man-days. See the file Audit_Mandays.doc (included) for an idea what you’re looking at.
A Systems Audit is an audit where high level company systems are reviewed. Typically we are talking about Level II procedures which form the backbone based upon the Quality Systems Manual.
Systems audits typically probe the interactivity (communication) of the inter-related company systems and as such often cross ‘functional’ area ‘boundaries’.
Typical Systems Audits:
Control of Measuring and Test Equipment
Systems Audits are typically carried out in multiple departments. For example, if one decides to audit Document Control, one must audit a number of departments.
ISO 19011 - Quality and Environmental Management Systems Auditing Forum Discussions