A decision-maker being impartial is one of the core requirements of natural justice.
In the 19th century, Lord Campbell in Dimes  EngR 789 said that “no man is to be a judge in his own cause”.
As a majority of the High Court noted in Ebner v The Official Trustee in Bankruptcy  HCA 63:
“Fundamental to the common law system of adversarial trial is that it is conducted by an independent and impartial tribunal. Perhaps the deepest historical roots of this principle can be traced to Magna Carta (with its declaration that right and justice shall not be sold)”.