British government ministers say Australia is holding up changes to the royal succession laws that would end the practice that sees sons take precedence over daughters.

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.

Every Commonwealth realm has now passed the law except for Australia, says Jim Wallace, the deputy leader of the British parliament's upper House of Lords.

The measures mean newborn male children would no longer overtake elder sisters in the line of succession.

Commonwealth realms rushed to agree to the changes after Prince William, the eldest son of Prince Charles and next in line to the throne after his father, got married in April 2011.

They were agreed in principle by prime ministers of the 16 countries at a Commonwealth summit in Perth on October 28, 2011.

The new laws will apply to any descendents of Charles, Queen Elizabeth II's eldest son, born from that date onwards but 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, Mr Wallace said all 16 realms agreed the legislation had to be put into place.

"All realms that took the view that legislation is required have passed the necessary legislation apart from Australia," he said.

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 said.

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.

However, Roman Catholics themselves remain excluded, the sovereign must be in communion with the Church of England.

The new laws would also limit the number of those in the line needing the monarch's permission to marry to six.

The other Commonwealth realms are Antigua and Barbuda, The Bahamas, Barbados, Belize, Britain, Canada, Grenada, Jamaica, New Zealand, Papua New Guinea, Saint Kitts and Nevis, Saint Lucia, Saint Vincent and the Grenadines, Solomon Islands and Tuvalu.

AFP