Novus actus interveniens
89. There is no evidence to suggest that Dr Hodge’s advice to proceed to surgery was unreasonable. Nor is there any evidence that Dr Hodge performed the surgery negligently. Even if he had, the employer would still be liable for the consequences of the surgery, unless the treatment provided was so inexcusably bad as to become a novus actus: Mahony v J Kruschich (Demolitions) Pty Ltd  HCA 37; 156 CLR 522 at 529–530. As observed by the High Court in Kruschich, with specific reference to workers compensation cases:
“the total condition of a worker whose compensable injury is exacerbated by medical treatment, reasonably undertaken to alleviate that injury, is to be attributed to the accident: see Lindeman Ltd v Colvin  HCA 35; (1946) 74 CLR 313, per Dixon J at p 321; Migge v Wormald Bros Industries Ltd (1972) 2 NSWLR 29, per Mason JA at p 48; on appeal (1973) 47 ALJR 236, although some medical negligence or inefficiency can be held to amount to a new cause of incapacity in some circumstances: Rothwell v Caverswall Stone Co (1944) 2 All ER 350, at p 365; Hogan v Bentinck Collieries (1949) 1 All ER 588, p 592.”
90. I am satisfied that Mr Kirkman acted reasonably in undergoing the surgery recommended by Dr Hodge and that any additional symptoms that he developed after the surgery have resulted from the surgery and, therefore, from the injury.
Cf March v E and M H Stramare Pty Ltd (1991) 171 CLR 506 per Mason CJ at 515-516, Deane J at 521 - 523, Toohey J at 524.
An instructive discussion is contained in the judgment of Mason CJ dealing with novus actus interveniens: ibid at 517-518, omitting most references:
"The facts of, and the decision in, M'Kew v Holland & Hannen & Cubitts  SC (HL) 20 illustrate the same deficiency in the "but for" test. The plaintiff would not have sustained his ultimate injury but for the defendant's negligence causing the earlier injury to his left leg.
His subsequent action in attempting to descend a steep staircase without a handrail in the normal manner and without adult assistance resulted in a severe fracture of his ankle. This action was adjudged to be unreasonable and to sever the chain of causation.
The decision may be explained by reference to a value judgment that it would be unjust to hold the defendant legally responsible for an injury which, though it could be traced back to the defendant's wrongful conduct, was the immediate result of unreasonable action on the part of the plaintiff.
But in truth the decision proceeded from a conclusion that the plaintiff's injury was the consequence of his independent and unreasonable action.
The fact that the intervening action is deliberate or voluntary does not necessarily mean that the plaintiff's injuries are not a consequence of the defendant's negligent conduct.
In some situations a defendant may come under a duty of care not to expose the plaintiff to a risk of injury arising from deliberate or voluntary conduct or even to guard against that risk: see Chomentowski v Red Garter Restaurant Ltd (1970) 92 WN (NSW) 1070.
To deny recovery in these situations because the intervening action is deliberate or voluntary would be to deprive the duty of any content.
It has been said that the fact that the intervening action was foreseeable does not mean that the negligent defendant is liable for damage which results from the intervening action ...
But it is otherwise if the intervening action was in the ordinary course of things the very kind of thing likely to happen as a result of the defendant's negligence.
In Dorset Yacht Co v Home Office  AC 1004, Lord Reid observed (at p 1030):
"But if the intervening action was likely to happen I do not think that it can matter whether that action was innocent or tortious or criminal. Unfortunately, tortious or criminal action by a third party is often the 'very kind of thing' which is likely to happen as a result of the wrongful or careless act of the defendant."
Much the same approach was adopted by this Court in Caterson v Commissioner of Railways (1973) 128 CLR 99 where Gibbs J (with whom Barwick CJ, Menzies and Stephen JJ agreed) pointed out (at p 110) that, if the plaintiff's action in jumping from the train was, in the ordinary course of things, the very kind of thing likely to happen as a result of the defendant's negligence and was not unreasonable, the jury was entitled to find that the plaintiff's injuries were caused by the defendant's negligence.
The finding that the plaintiff's action was not unreasonable was then essential to that conclusion because contributory negligence was a defence in New South Wales at the relevant time ...
As a matter of both logic and common sense, it makes no sense to regard the negligence of the plaintiff or a third party as a superseding cause or novus actus interveniens when the defendant's wrongful conduct has generated the very risk of injury resulting from the negligence of the plaintiff or a third party and that injury occurs in the ordinary course of things.
In such a situation, the defendant's negligence satisfies the "but for" test and is properly to be regarded as a cause of the consequence because there is no reason in common sense, logic or policy for refusing to so regard it.