Prime Minister Malcolm Turnbull has snubbed a 'royal' invitation — but it is not likely to cause a diplomatic incident.

On Saturday, many oceans away from Buckingham Palace, Prince Leonard — the self-declared sovereign of the Principality of Hutt River — will cede the throne.

Mr Turnbull was one of several dignitaries on the guest list, but has sent his regrets.

"Thank you for your invitation to the Prime Minister, the Hon Malcolm Turnbull MP, to attend the Abdication ceremony of Prince Leonard of the Principality of Hutt River," the RSVP from Mr Turnbull's office reads.

"Unfortunately, the Prime Minister is unable to attend your event due to existing parliamentary commitments."

Prince Leonard is 92 and battling emphysema.

While Prince Leonard is set to give way, memories of his reign remain in plain sight. ( ABC News: Eliza Borrello )

After almost five decades on the throne, he said it would be a "mental relief" to hand over the principality, 500 kilometres north of Perth, to his 59-year-old son Prince Graeme.

"I don't think I'm at 92 the most presentable person to be talking to tourists. I think some of the young ones are more presentable," he said

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Listen Duration: 3 minutes 4 seconds 3 m 4 s Listen to Eliza Borrello's full story here Download 5.6 MB

The former grain farmer claims to have seceded from Australia in 1970 over a dispute about wheat quotas.

"I raced to Perth to have a look at the Act of Parliament, when I got to Perth found they didn't have one," he said.

"Sir Charles Court went over to help the Minister for Agriculture who was advising the Governor, he returned to parliament, introduced a bill whereby under that bill he could get the power to resume our lands. I then exercised the international law entitlements to form a self-preservation government."

Paperwork will be signed by Prince Leonard during the ceremony to confirm his abdication. ( ABC News: Eliza Borrello )

Prince Leonard's many battles

Over the following 47 years, Prince Leonard issued his own currency, declared war on Australia and set up a Royal College of Advanced Research.

"I was going to set up a big research organisation in Jordan, through my representative there. Australian Government found out and they put pressure on Jordan's Government," the self-taught physicist said.

"And my chappy got word that he was about to be arrested and so he had to flee, so that stopped that."

Prince Leonard has seldom left the principality, but admits "sneaking" into the nearby towns of Northampton and Geraldton for medical treatment.

He has also refused to pay tax.

Just last week the tax office issued him with a writ, claiming he owes it $2.6 million.

It is a battle his son Graeme will take up when he ascends the throne.

"I'm going to contend it. We'll be entering our plea and putting paperwork into the court by the middle of February," he said.

The Principality of Hutt River, 500 kilometres from Perth, is Australia's oldest micro-nation. ( ABC News: Eliza Borrello )

Cucumber sandwiches to mark occasion

Prince Graeme said the ATO's main beef was over the non-collection of GST from tourists who bought souvenirs.

The principality sells Hutt River coins, stamps, caps and stubby holders, and people can even get their passport stamped.

Prince Graeme, holding a Hutt River passport, is ready to take up his father's battles. ( ABC News: Eliza Borrello )

The royal family issued an open invitation for the abdication ceremony and has prepared to welcome about 150 visitors.

Prince Graeme was still busily processing RSVPs when the ABC visited, and was not surprised the Prime Minister sent his regrets.

"They're very busy ... the Premier of Western Australia's also very busy with the state election coming up, so perhaps he might be busy that weekend as well," he said.

But Prince Graeme said he wanted anyone who had not confirmed their attendance to do so as soon as possible, so he could "make sure there's enough cucumber sandwiches."