it's just a clerical check, they check that all pages were scanned onto the marking system correctly, etc. it is very rare for an error to be found as hsc marking has many quality control procedures, especially because it is done online. for example, if you used an extra writing booklet but it was not scanned in, the contents of the booklet would most likely increase your markshow are they going to though they’re not actually re marking everything and it’s not like missing a page will cause your marks to be higher
Schools can request an investigation through the Anomalous Results Inquiries process if the marks for a student or group are much lower/different from expected. Again however, it is very unusual for anything to be changed as it is not an appeal or remark, instead more of an inquiry and investigation