The 16 Commonwealth realms which share the same royal family - including Britain, Australia, Canada, New Zealand and Jamaica - must all pass an identical law before the changes can come into effect, unless UK legislation covers it for them.
They have all done so except Australia, said Jim Wallace, the deputy leader of the British parliament's upper House of Lords, yesterday.
The measures mean newborn male children would no longer overtake elder sisters in the line of succession.
They were agreed in principle by prime ministers of the 16 countries at a Commonwealth summit in Perth, Australia on October 28, 2011.
The new laws will apply to any descendants of Charles -- Queen Elizabeth II's eldest son -- born from that date onwards. They can only come into effect when all the realms have ratified it.
Questioned in the House of Lords as to whether any realm might default on the Perth agreement, Wallace told peers: "The intention is that when all 16 realms have agreed or put in place the necessary legislation there will be a simultaneous order to give effect in each of the realms.
He said the six Australian states would legislate first before Canberra does.
"To date, three states have enacted legislation, two have introduced legislation and one, South Australia, has yet to introduce legislation but it is in the middle of an election campaign," he added.
The law change must be unanimous and identical in each realm to avoid the possibility of ending up with different monarchs in different countries.
In other changes, the new law would also state that anyone marrying a Roman Catholic no longer loses their place in the line of succession.