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)”.
Legal meaning of biased
Bias in the law is a decision-maker making a decision because of factors other than the merits of the case, rather than different treatment of the parties.
Therefore, the fact that a decision-maker disbelieves, or is critical of, a party or a witness does not, of itself, indicate bias: Ramadan v New South Wales Insurance Ministerial Corporation, unreported, New South Wales Court of Appeal, 7 April 1995 (“Ramadan“) at 3 per Kirby P, at 11 per Rolfe AJA. Furthermore, it is permissible for decision-makers to make known their preliminary views for the purpose of ventilating issues. Even where preliminary views are strongly held, there will be actual bias only if they are incapable of being altered. Also, it is not always sufficient to show that a decision-maker has displayed irritation or impatience or has even expressed sarcasm, and that whether such matters show actual bias remains a question of fact: see Galea v Galea (1990) 19 NSWLR 263 at 279 per Kirby ACJ.
“requirements of natural justice are not infringed by a mere lack of nicety but only when it is firmly established that a suspicion may reasonably be engendered in the minds of those who come before the tribunal or in the minds of the public that the tribunal or a member or members of it may not bring to the resolution of the questions arising before the tribunal fair and unprejudiced minds. Such a mind is not necessarily a mind which has not given thought to the subject matter or one which, having thought about it, has not formed any views or inclination of mind upon or with respect to it.”