(a) That's wrong. The necessary rules for the internal administration and management of a multi-director/shareholder company can be provided for in more than one way. The Corporations Act 2001 (Cth) provides a number of what can be called 'residual' or 'replaceable' rules of internal governance. These rules govern questions such as how directors are to be appointed, or how meetings are to be run. The rules are called 'replaceable' because they can be replaced by different rules that an individual company might prefer.
If an individual company wants to govern itself in accordance with different rules from those laid down in the Corporations Act as 'replaceable' rules, they may do so. The chosen rules are set out in a document called a 'constitution'. The extent to which a particular company is content to be governed by the replaceable rules, or prefers to adopt its own constitution, is a matter that depends on the circumstances. For the present, it is important to understand that the internal affairs of a company can be regulated either by rules laid down in the Corporations Act, or by the rules contained in the company's own constitution, or by a combination of both.
These rules have the effect of contractual terms agreed to by the company and its members; the company and its officers; and between the individual members of the company. The rules give rise to private rights and duties between these persons, and are enforceable on that basis.These rules are not enforceable by other persons.
Before 1998, companies were internally governed according to the provisions contained in documents called their 'memorandum of association' and 'articles of association'.
Companies formed before 1998 continue to be governed by the provisions of these documents, so you may still encounter mention of them. The best approach is to think of it as though the provisions of the memorandum and articles have been adopted in place of the 'replaceable terms'.
Of course it is possible for a company that was created before 1998 to decide to do away with its memorandum and articles, and instead adopt the replaceable terms, in whole or in part.