This is a significant concession the state makes to the independence of the judiciary and essentially so that judges may make the difficult decisions that are often required without fear of being sued: fiat justitia ruat caelum.
It does not cover judges from the consequences of crimes they may commit even if committed during the course of their official action; the immunity is for civil liability only.
A good example was Royer v Mignault in which a judge slandered a lawyer by making a derogatory remark about his abilities. The lawyer sued but the dumb judge was found to be immune from the net of defamation liability because the utterance was made during a court hearing.
The immunity, according to Robert Kligman's 2011 article, covers "almost anything that transpires in a judicial proceeding".
Common law lawyers often harken back to the words of Lord Denning in Sirros v Moore:
"What is the test upon which the judges of the superior courts are thus immune from liability for damages even though they are acting without jurisdiction? Several expressions are to be found. A judge of a superior court is not liable for anything done by him while he is "acting as a judge," or "doing a judicial act" or "acting judicially" or "in the execution of his office"....
"What do all these mean? They are much wider than the expression when he is acting within his jurisdiction. I think each of the expressions means that a judge of a superior court is protected when he is acting in the bona fide exercise of his office and under the belief that he has jurisdiction, though he may be mistaken in that belief and may not in truth have any jurisdiction. No matter that his mistake is not one of fact but of law, nevertheless he is protected if he in good faith believes that he has jurisdiction to do what he does."
To borrow from Justice Allen Sharp of the United States District Court (Indiana) in Sims v Kernan:
"The doctrine of judicial immunity affords state judges absolute immunity for past judicial acts regarding matters within their court's jurisdiction, even if their exercise of authority is flawed by the commission of grave procedural errors."
It covers judges and, in most cases, all others involved in judicial decision-making such as the members of administrative tribunals.
In Quitoriano v Raff & Becker, Justice Chin, also of the U.S. District Court (New York), wrote:
"Under the doctrine of judicial immunity, judges and certain judicial authorities are exempt from civil liability for acts undertaken in furtherance of their judicial functions. The doctrine is based on a fear that exposure to potential damages liability might impair the impartiality and independence of the judiciary.
"The touchstone for the doctrine's applicability has been performance of the function of resolving disputes between parties, or of authoritatively adjudicating private rights....
"Judicial immunity extends to individuals beyond the judiciary when their actions are functionally comparable to those of judges. At common law, judicial immunity extended to certain private citizens, particularly jurors and arbitrators. Other examples of persons whose functions have been deemed quasi-judicial include commissioners appointed to appraise damages when property is taken under the right of eminent domain; members of town boards in determining the allowance of claims; prosecutors; and law clerks. Court reporters are not protected by judicial immunity because their function is not sufficiently adjudicatory."
Judicial immunity is often set by statute, in which case reference should be made to that standard. For example:
" ... no legal proceeding for damages lies or may be commenced or maintained against a decision maker, the tribunal or the government because of anything done or omitted
(a) in the performance or intended performance of any duty under this Act or the tribunal's enabling Act, or
(b) in the exercise or intended exercise of any power under this Act or the tribunal's enabling Act.
(This) does not apply ...relation to anything done or omitted by that person in bad faith."1
- Administrative Tribunals Act, SBC 2004, c 45, §56 [NOTE 1]
- Kligman, Robert, Judicial Immunity, 38 Ad. Q. 251 (2011)
- Quitoriano v. Raff & Becker, 675 F. Supp. 2d 444 (2009)
- Roger v Mignault, 50 DLR (4th) 345 (1988; Québec)
- Sims v. Kernan, 29 F. Supp. 2d 952 (1998)
- Sirros v. Moore,  3 All ER 776