According to my 11th edition Britannica, the common informers
or delators of ancient Rome were a class of private citizens who specialized in bringing accusations against others: "They were drawn from all classes of society - patricians, knights, freedmen, slaves, philosophers, literary men, and, above all, lawyers." The right to file charges against a fellow citizen was not in itself new, but took on a new character when the state began awarding the delator a share of the property of the accused; a successful accusation of treason, for example, carried as a prize a quarter of the victim's estate.
The matters considered by the High Court were preliminary questions of law relating to whether it could and should decide the eligibility of a Member of Parliament under the Common Informers
Act, rather than the long standing process of such decisions being made under section 47 of the Commonwealth Constitution and the Commonwealth Electoral Act 1918.