65 It maybe given, I think, and as the cases show, that it is generally more difficult to establish negligence in an employer from a system of work rather than as a result of a discrete incident in the workplace. And that is so because the system of work may well be safe for the average worker but unsafe for a particular worker due to, say, a constitutional condition aggravated by the work and where no discernible act or omission of the employer be shown to exist. Where injury be alleged, as here, from the nature and conditions of the work over a period of time with the presence of a pre-existing congenital fault it may be admitted that the difficulty in showing negligence is increased. That, in short, was the position taken by the present defendant. However, at the end of the day it is a question, in my view, of examining all the circumstances in a particular case to ascertain the true position. It is to be borne in mind also that liability in an employer arises where a plaintiff's injury is not only caused but is materially contributed to by the negligence of the defendant: see Purkess v Crittenden (1965) 114 CLR 164 and Tubemakers of Australia v Fernandez (1976) 50 ALJR 720; (1976) 10 ALR 303. It is sufficient in establishing negligence in the employer to show that its wrongful act may not be the sole cause of any injury as long as it is a contributing cause which is more than negligible; see Chapman v Hearse (1961) 106 CLR 112. And, as Purkess v Crittenden (114 CLR at 166 in para [4]) made clear:
"...where a plaintiff has, by direct or circumstantial evidence, made out a prima facie case that incapacity has resulted from the defendant's negligence, the onus of adducing evidence that his incapacity is wholly or partly the result of some pre-existing condition or that incapacity, either total or partial, would, in any event, have resulted from a pre-existing condition, rests upon the defendant. In other words, in the absence of such evidence the plaintiff, if his evidence be accepted, will be entitled to succeed on the issue of damages and no issue will arise as to the existence of any pre-existing abnormality or its prospective results, or as to the relationship of any such abnormality to the disabilities of which he complains at the trial ... "