Michael Nibbs contended so many different very common Australian pseudo legal themes in this case it is among my favourites, but I suppose the main point is the objection raised regarding judicial oath of office.
Before the Promissory Oaths Act 2015, Magistrates and Justices were required to take and subscribe the judicial oath prescribed by the Promissory Oaths Act 1869. Section 7 required the oath to be tendered by the Clerk of the Executive Council and taken in the presence of the Governor or such specified person as the Governor directed. In Nibbs v Devonport City Council  TASSC 34, Michael Nibbs produced a copy of a media report of a statement made by the Attorney-General on or around 3 April 2015. The Attorney is reported to have said that the requirement for the judicial oath under the 1869 Act was outdated, “and it was reasonably established that it had not been followed for the past three decades”.
Porter J considered whether a Magistrate had authority to hear case if not correctly sworn into office, and also considered the validity of Local Government Act 1993 (Tas). Michael Nibbs asserted that there was no valid Royal Assent as the State Governor had not properly been sworn in. It was held that section 45A of the Constitution Act 1934 (TAS) establishes Local Government in Tasmania, and section 120 of the Local Government Act 1993 (TAS) established Devonport Council.
Further, that section 21 of the Promissory Oaths Act 2015 (Tas) retrospectively validates judicial oaths or affirmations, oaths or affirmations of allegiance and oaths or affirmations of office, with the result being the Governor and Magistrate had been correctly sworn in under this Act.
Additionally, Porter J relied on the presumption of regularity and de facto officer doctrine.
“The principle commonly known as the ‘presumption of regularity’ is that where the exercise of a power or the performance of an act by a public officer or public authority is proved, it will be presumed that the preconditions to the lawful exercise of that power or performance of that act have been met.”
McLean Bros & Rigg Ltd v Grice  HCA 1; (1906) 4 CLR 835 at 560. Relevant to this case, in Minister for Natural Resources v New South Wales Aboriginal Land Council (1987) 9 NSWLR 154, McHugh JA at 164 said:
“Where a public official or authority purports to exercise a power or to do an act in the course of his or its duties, a presumption arises that all conditions necessary to the exercise of that power or the doing of that act have been fulfilled. Thus a person who acts in a public office is presumed to have been validly appointed to that office.”
(M’Gahey v Alston  EngR 150; (1836) 2 M & W 206 at 211;  EngR 150; 150 ER 731 at 733; R v Brewer  HCA 33; (1942) 66 CLR 535 at 548; Hardess v Beaumont  VicLawRp 46;  VLR 315 at 318-319.)
The second answer is that in any event, the doctrine known as the ‘de facto officer doctrine’ would undoubtedly apply. G J Coles v Retail Trade Industrial Tribunal (1986) 7 NSWLR 503 per McHugh JA at 515:
“The acts of a de facto public officer done in apparent execution of his office cannot be challenged on the ground that he has no title to the office. It matters not that his appointment to the office was defective or has expired or in some cases even that he is a usurper.”
As his Honour demonstrated in his analysis in that case, and as demonstrated in the discussion by Crawford J (as he then was) in Official Trustee v Byrne  Tas SR 1 at 13-15, the principle applies to judicial officers.
The three conditions necessary for the operation of the doctrine apply in this case. The office of a magistrate is one which existed in law. The acts of the magistrate in hearing and determining the application for summary judgment were within the scope and authority of the office of a magistrate. Lastly, the doctrine should properly be applied in the public interest: see generally Jamieson v McKenna  WASCA 325 at - .”