What is merger in judgment and its relationship with Res judicata?

A res judicata is a decision given by a judge or tribunal with jurisdiction over the cause of action and the parties, which disposes, with finality, of a matter decided so that it cannot be re-litigated by those bound by the judgment, except on appeal.

For further guidance, see Practice Note: The doctrine of res judicata.

Merger in judgment is a doctrine that treats a cause of action as extinguished once judgment has been given on it so that the claimant's sole right is a right on the judgment. It is a substantive rule about the legal effect of an English judgment, which is regarded as 'of a higher nature' so that the inferior remedy is merged in the higher and therefore supersedes the underlying cause of action (King v Hoare).

Autrefois acquit and autrefois convict enforce the doctrine of merger in judgment in the criminal law (Rogers v R (1994) 181 CLR 251 (at pp 256, 279) (not