3 (b) That's wrong. It does not seem possible to define with complete certainty what events will be recognised by the courts as a novus actus interveniens. In addition to the conceptual difficulties involved, the courts clearly use the concept to give effect to policy considerations, particularly to limit or extend a defendant's liability in a particular case as seems appropriate.

For example, even careless medical treatment that further harms a person injured by the defendant is not treated as a novus actus interveniens unless that treatment was 'inexcusably bad'. This means that a defendant is not too easily relieved of responsibility. See Mahony v J Kruschich (Demolitions) Pty Ltd (1985) 156 CLR 522.

For this reason, any question of novus actus interveniens must be approached with great caution and a measure of common sense.