In 1822, he was appointed to be Chief Justice of the Supreme Court of New South Wales, to oversee the reform of the administration of the legal system in the colony, following the inquiry into the colony's affairs by commissioner John Bigge. Before departing for Australia, he helped draft the New South Wales Act 1823 (4 Geo. IV c. 96) which, along with the Charter of Justice issued under it on 13 October 1823, replaced the legal tribunals of convict days with a Supreme Court possessing comprehensive jurisdiction. Under the new system, Forbes was not only the sole judge, subject only to the appellate power of the Governor, but also an "ex officio" member of the Executive Council and the Legislative Council, and all colonial legislation had to be certified by him as not being repugnant to the laws of England.