156 The principles governing appellate review of a primary judge’s apportionment of culpability between two tortfeasors were explained in Ghunaim v Bart  NSWCA 28; (2004) Aust Torts Reports ¶81-731:“45 Appellate review of a trial judge’s apportionment of liability as between respectively culpable parties, however, is governed by the stringent tests which limit appellate review of discretionary decisions.
46 In Podrebersek v Australian Iron & Steel Pty Limited  HCA 34, (1985) 59 ALJR 492 at 493 – 494 Gibbs CJ, Mason, Wilson, Brennan and Deane JJ in their joint judgment said: “[a] finding on the issue of apportionment is a finding upon a ‘question, not of principle or of positive findings of fact, or law, but of proportion, of balance and relative emphasis, and of weighing different considerations. It involves an individual choice or discretion, as to which there may well be differences of opinion by different minds’ …such a finding, if made by a judge, is not lightly reviewed.
47 This Court has frequently reminded itself of the necessity to exercise restraint in reviewing a trial judge’s decision on apportionment. As Sheller JA (with whom Powell and Heydon JJA agreed) said in Mousa v Marsh  NSWCA 317 at :
“It is well established that since a court’s apportionment of the degree of liability due to the plaintiff’s contributory negligence is a question not of principle or of positive findings of fact or law but of proportion, of balance and relative emphasis and of weighing different considerations all of which involve an individual choice or discretion as to which there may well be differences of opinion by different minds (see British Fame (Owners) v Macgregor (Owners)  AC 197 at 201), such a finding, if made by a judge, is not lightly reviewed.”
48 To set aside an apportionment of liability it must be shown that the failure to exercise the discretion involved in the apportionment exercise properly was unreasonable or plainly unjust: Rolls Royce Industrial Power (Pacific) Limited v James Hardie & Co Pty Limited (2001) 53 NSWLR 626 at 637  per Stein JA with whom Davies A-JA agreed (655 at ). Fitzgerald A-JA was of the view that an appeal court was not entitled to interfere “if the trial judge’s apportionment was reasonably open.” (653 at ). Rolls Royce was appealed to the High Court as Amaca Pty Ltd v State of New South Wales  HCA 44, (2003) 77 ALJR 1509. As Tobias JA (with whom Sheller and Foster AJA agreed) recently observed, the appeal was allowed on an issue which did not affect the correctness of Stein JA’s statement of principle: Rexstraw v Johnson  NSWCA 287.”