The critical point was that the court held that the word “marriage” in the Constitution means the union of any two natural people. It was not restricted to unions between a man and a woman.

As it happens, the 1961 Federal Marriage Act restricts marriage to unions between a man and a woman, but the court held that it would be open to the Federal Parliament to change that and legislate for same-sex marriages under the marriage power in the Constitution.

Hitherto the point was unresolved. It was thought that if ever the Federal Parliament did legislate for same-sex marriage it would invite a constitutional challenge on the basis that the word “marriage” in the Constitution meant only unions between a man and a woman.

That argument will now no longer run and any future Federal law providing for same-sex marriages would be within the Commonwealth Parliament’s power and a challenge would be futile.

So in the long run, the case improves the prospect of a national law for same-sex marriages and has put paid to any move by any other state or territory to provide for them.

In a way the result is a good one for marriage equality. Having same-sex marriages under state and territory law and other marriages under federal was never going to be satisfactory.

Thursday’s decision paves the way eventually for a national law.

Before Thursday’s decision no-one knew whether the word “marriage” in the 1901 Constitution would enable the Federal Parliament to make laws for same-sex marriages.

We now know that it does.

The court has not yet published its full reasons, but the summary put up on the court’s website on Thursday indicates at least some of its reasoning.

The court went beyond what the Commonwealth had sought at the hearing. At the hearing the Commonwealth Solicitor-General, Justin Gleeson SC, had argued that it was unnecessary for the court to decide the question of the meaning of the word “marriage”. He said it did not matter whether it had a wide meaning or a narrow meaning. The Commonwealth Marriage Act covered the field either way and left no room for any state or territory marriages.

He said the Commonwealth Marriage Act provided who was eligible to change their marital status from unmarried to married and it provided the comprehensive means of determining which people in Australia had the status of being married and which had the status of being unmarried.

There was no room for any state or territory law to alter that line.

There is no telling what Mr Gleeson’s instructions from the Commowealth Government were. Perhaps they were to run neutral of the question of whether the word “marriage” in the Constitution included same-sex marriages.

In any event, the court did not run neutral on the question. It has clearly found it necessary to determine the issue of the meaning of the word “marriage” in the Constitution in order to work out whether the ACT law is invalid or conflicts with the Federal law.

If it conflicts (which the courts said it did) then it is of no effect while ever the Federal law stays on the statute book.

The decision was unanimous. Six of the seven judges sat on the case. They found that the word “marriage” in the Constitution means the union of any two natural persons.

This will be a major blow to religious groups who would have no doubt challenged any Federal same-sex marriage law. Now they have no grounds to do so.

The case is not what it seems at first. It is in fact a major step forward for marriage equality in Australia.