The oldest micro-nation in Australia may soon enter a new era, as its 90-year-old monarch contemplates retirement.

Based on an agricultural property 500 kilometres north of Perth, the Principality of Hutt River was born out of a farming family's stoush with the State Government.

Through a series of legal manoeuvres, Leonard George Casley - who calls himself Prince Leonard - founded the principality in 1970 in response to the WA Government's wheat production quotas, which would have allowed only a fraction of his farm's crop go to market.

"If we would have accepted that quota, it would have made the farm at that time valueless," Prince Leonard said.

The principality, which the Australian Government does not legally recognise, has existed for 45 years.

The unusual enclave, based near Northampton, attracts tourists from around the globe, who make the pilgrimage to have their passports stamped and purchase the local currency, the Hutt River dollar.

The Principality of Hutt River, which has its own flag and official seal, has not always been peaceful.

In 1977, following repeated demands for payments from the Australian Taxation Office (ATO), the province declared war on Australia.

It withdrew several days later and is treated by the ATO as a non-resident of Australia which does not pay taxes but is believed to give annual "gifts" to the local shire.

A self-described mathematician and physicist, Prince Leonard recently celebrated his 90th birthday.

He said poor health may soon prompt his retirement, and he may live to see his successor take up the role.

"I may have to resign shortly and then might leave that to happen in my lifetime, rather than when I'm gone," he said.

"I want to see a few more projects, the projects that I'm after, I want to see them done.

"But providing that I don't get any worse than what I am now... I'll keep going for a while.

"But should I deteriorate a little bit ... I'm quite prepared to sit aside and watch."

A decision has not been made on which of his seven children will succeed him.

The principality of Hutt River has a crown committee made up of Prince Leonard's three daughters, who could be called upon to determine their father's successor.

One of his sons, Prince Graeme, said any succession may still be years away.

"Whilst he celebrated his 90th birthday on the 27th of August, he is full of energy, full of projects and he just keeps going on and on," he said.

"We're very lucky and look forward to another 90 years ... at least 10, so he can send a birthday card to himself."

Prince Leonard has privately published a number of research papers and books and intends to further develop an educational shrine that he dedicates to his wife, Princess Shirley, who died in 2013.

"That will be his legacy to us," Prince Graeme said.