If the Scottish independence vote is successful the political and legal ramifications could flow to Australia, leaving it without a head of state, a constitutional expert has warned.

Associate Professor Iain Stewart from Macquarie University said a 'Yes' vote in Scotland could "set adrift" the Queen's representatives in Australia.

"It could remove our head of state, the monarch, and thus, at least legally, make government in Australia impossible," he said.

"We need to think ahead. We may need to move to a republic and soon."

Scots will answer the question "Should Scotland be an independent country?" in a referendum on September 18.

Australian Prime Minister Tony Abbott said supporters of Scottish independence were not "friends of freedom" or "friends of justice" in comments that were the strongest yet by a foreign leader on the independence debate.

Mr Abbott's comments drew the ire of Scotland's first minister Alex Salmond, who said the Australian leader had "put his foot in it".

Mr Salmond is the leader of the Scottish National Party (SNP), which wants Scots to vote Yes in the September vote.

Revival of Scottish monarchy central to Australian implications

Professor Stewart said if Scotland regained its independence, a revival of the Scottish monarchy appeared to be a likely option.

A white paper released by the Scottish government late last year suggested the Queen would remain as an independent Scotland's head of state.

That could fix an issue in Scotland, but create a serious one in Australia.

"The Yes camp still doesn't seem to have clarified what would happen to the monarchy," Professor Stewart said.

"They may mean a revival of the Scottish monarchy, inviting Queen Elizabeth II to take it on, so that there would once again be a personal union of the crowns."

Professor Stewart said that meant "a single person [would sit] on both thrones of the [separate] monarchies of Scotland and [the] remaining Britain - as there was for Scotland and England before the two countries united under a single throne in 1707".

He said if that occurred, then "there would no longer be any sovereignty of the United Kingdom on which Australia could draw for its own head of state".

"Whatever Queen Elizabeth II or her successor may do, they would not occupy a throne of the 'United Kingdom', which is how Australian constitutional law defines our head of state."

Professor Stewart said that would "set adrift" the roles of Australia's governor-general and the governors of each Australian state.

"This would affect the whole apparatus of the government," he said.

Professor Stewart warned Scottish independence could also pose a challenge to the validity of the governor-general, who alone can summon Federal Parliament.

This could mean the Federal Government would only be able to continue until the end of the next parliamentary session.

If there was the sudden need for a general election, this would also be a problem because only the governor-general can formally call one.

The governor-general's roles also include appointing all government ministers, justices of the High Court and other federal courts, and assenting to legislation passed by Federal Parliament.

"Still worse, when the present governor-general's term expires (presumably in March 2019), we would not know of a 'Queen' to reappoint him or appoint a successor."

'Fancy legal footwork' may be required

Professor Stewart said fixing the potential problem of sovereignty "would require some pretty fancy legal footwork" in Australia, and more broadly the consultation of the "Commonwealth realms", the 16 countries, including Australia, that Queen Elizabeth II heads.

He said the complexity was demonstrated in the recent attempt to change royal succession laws that see royal sons take precedence over royal daughters.

Every Commonwealth realm has passed the law except Australia.

Because the Federal Government has no explicit power to legislate about the monarchy, it first had to wait until all six states had passed the changes. The bill was delayed in South Australia due to the March state election and was passed in that state in June.

Professor Stewart said it would be highly undesirable for issues surrounding Scottish independence to end up in the High Court of Australia or for legislation to fix the sovereignty issue to be delayed because the legislation required sovereign assent.

Separate Australian monarchy, republic possibilities

One solution may be to adopt legislation similar to what the British government may devise.

But Professor Stewart said that would be a risky path.

Instead, he said a separate Australian monarchy could be created, or the monarchy could be abolished through a new referendum on a republic for Australia.

The last referendum was held in 1999, when the propositions failed.

On the issue of the Australian flag, Professor Stewart said no change would be required because of Scottish independence.

"We can keep the Union Jack in our flag whatever the British may do with it," he said.

Any changes to the Australian flag must go to a national referendum.

"Nor would it require us to remove Her Majesty from the $5 note, though maybe an Australian should adorn it anyway," Professor Stewart said.

For Australians born in Scotland, or with Scottish parents or grandparents, Scottish independence would mean they become entitled to Scottish citizenship.