When a person fails to act in accordance with a duty of care, their breach of duty may result in harm. Whether a particular breach of a duty causes particular harm is a question of fact.
In some cases it is not difficult to conclude that the harm suffered by a plaintiff is the result of the defendant's breach of duty. In such cases we can say the defendant's breach of duty is 'causally linked' to the plaintiff's harm.
In other cases, the link between the defendant's conduct and the harm suffered is not so obvious. In such cases the plaintiff must prove that the defendant's breach of duty was a necessary cause of the particular harm suffered by the plaintiff. Otherwise the defendant will not be liable in Negligence for that harm.
Adeels Palace Pty Ltd v Moubarak; Adeels Palace Pty Ltd v Bou Najem (2009) 239 CLR 420.