4 (a) That's probably wrong. In the 19th century, the law assumed that employees accepted the risks involved with their employment. Accordingly, they could not claim damages for injuries suffered. These days it is more commonly thought that an employee's consent to risks is not freely given.
This restricts the availability of volenti non fit injuria as a defence for employers. However it has not disappeared completely (see for example Imperial Chemical Industries Ltd v Shatwell [1965] AC 656), except in NSW by virtue of the Workers Compensation Act 1987 (NSW). See Joslyn v Berryman; Wentworth Shire Council v Berryman [2003] HCA 34.