"The law with respect to parity is settled, at least in the ordinary case. Speaking generally, justice should be equal and like offenders should be treated alike: Postiglione v The Queen [1997] HCA 26; (1997) 189 CLR 295 at 301, per Dawson and Gaudron JJ. Equal justice requires that, as between co-offenders, there should not be a marked disparity which gives rise to a justifiable sense of grievance: Lowe v The Queen [1984] HCA 46; (1984) 154 CLR 606 at 617 - 618, per Brennan J, and Postiglione, ibid. For a sense of grievance to be justifiable, the difference between the sentences must be manifestly excessive: Lowe, at 624, per Dawson J. Moreover, if there are factors which support different treatment as between co-offenders, whether because of different degrees of culpability or differences in their circumstances, then, of course, it will be appropriate to treat them differently: R v Cox (1996) 66 SASR 152, per Doyle CJ, and Postiglione, at 301, per Dawson and Gaudron JJ. In such a case the difference in sentence, if it is a reasonable consequence of the different factors, can give rise to no justifiable sense of grievance.
In considering the application of the parity principle, sight should not be lost of the community interest in seeing offenders appropriately punished. The imposition of an excessively lenient sentence on one co-offender does not have the automatic consequence that a similar error should be made in the case of another. While parity might require a sentencing Judge to be more lenient than he or she might otherwise have been, it does not require a sentencing Judge to be so lenient as to shock the public conscience by imposing a sentence which is entirely disproportionate to the offence."