At the moment the law prefers males over females. So even if a boy is delivered second, he would still precede his older sister in the line of succession. That archaic and sexist law might well soon change.

At the meeting of the Commonwealth Heads of Government in Perth in 2011, all the countries of which the Queen is head of state, known as ''realms'', agreed that the rules should be changed so that males would no longer be given preference over females and heirs would no longer be disqualified from succeeding to the throne as a consequence of marrying a Catholic. It was generally agreed that discrimination on the grounds of sex and religion are no longer appropriate today (although the monarch would still have to be ''in communion with the Church of England'' and discrimination based upon family lineage would be retained).

"The problem of timing would be resolved if Kate had a first-born son". Photo: Getty Images

Agreement in principle is one thing, but making the constitutional change is another. In the old days, the Westminster Parliament would simply enact a change to the law of succession and it would apply to all the realms. But these days, Britain can no longer legislate for any of the 15 other realms, including Australia. Each of them must make its own changes to the law of succession of its own crown, by whatever constitutional method is required. New Zealand has the unenviable task of trying to co-ordinate this international process, but has discovered that many of the smaller Realms have limited resources and simply do not see this as a priority, given their more significant problems. So the process of change is not proving to be a speedy one.

In Australia, the issue is complicated by the federal system. It is most likely that the Federal Parliament does not have unilateral power to change the rules of succession. It has no specific power to do so and as the Queen is also a part of state constitutions and state parliaments, it is likely that any Commonwealth attempt to interfere with state constitutions would be invalid. The most appropriate approach is to use a section of the constitution that permits the states and the Commonwealth to co-operate to enact laws that only the Westminster Parliament could have enacted at the time of Federation. This would entail each state parliament passing a law that requests the Commonwealth to enact the changes to the Australian law of succession.

It is fairly unlikely that any state would have a philosophical objection to making changes that remove discrimination on the basis of sex and religion. The states all have anti-discrimination laws and it has long been incongruous that the sovereign is chosen by discriminatory methods that would otherwise breach Australian laws. However, in the context of current Commonwealth-state relations, it would not be too surprising if at least one state held out,until it squeezed some benefit or promise from the Commonwealth relating to other issues of inter-governmental conflict.

The problem for Britain is that while it can legislate to change the rules of succession in relation to its own throne, it does not want to risk the bifurcation of the throne if some of the realms do not make the same constitutional change. It would not want to risk the throne of the United Kingdom and New Zealand being held by the eldest daughter of William and Kate, while the throne of Canada and Australia was held by her younger brother. Hence its plan has been to introduce legislation to make the change, but provide that it does not come into force until the same change is made in all the other realms. This may take quite a while.

The problem of timing would be resolved if Kate had a first-born son. With three males in direct line of succession, the urgency of sorting out the rule changes is significantly diminished. But what if Kate had twins, with a girl delivered first and then a boy? Although unlikely, this could potentially lead to difficulties. First, as a matter of law, the boy would be legally third in line to the throne, displacing his older sister. Although it was agreed at CHOGM that any laws enacting these changes would apply retrospectively to all heirs born after 2011, the boy would still be third in line to the throne until such a change was legally made. The problem then would be that each of the realms would presumably change their laws at different times, with the daughter progressively upgrading her place in the line of succession as her brother was progressively demoted (unless all realms agreed on a uniform commencement date). For the younger brother, this might prove to be a case of heir today and gone tomorrow.

Anne Twomey is a professor of constitutional law at the University of Sydney.