(a) No. The constitution of the Commonwealth of Australia is contained in a law enacted by the parliament of the United Kingdom - the Commonwealth of Australia Constitution Act 1900. The provisions of this constitution cannot be easily changed. To do so requires obtaining the consent of Australian voters in a national referendum. The most recent attempt to change the Commonwealth constitution and make Australia a republic did not succeed.
By contrast, the state governments have enacted their own constitutions (acting in terms of a power granted by the United Kingdom parliament). The relevant government can change state constitutions without the need for a special referendum.