King Charles' antipodean admirers got a first glimpse of their reigning monarch Sunday as the British royal attended a church service and expressed his "great joy" at returning Down Under.

The 75-year-old sovereign arrived in Sydney late on Friday evening but had kept a low profile as he balanced cancer recovery with royal duties.

His first official public appearance was a Sunday morning service at St. Thomas' Anglican Church, a stone edifice built as a place of worship for British colonial settlers.

A few hundred people gathered around the building, cheering, holding flowers, and waving flags. Two women held up a sign saying "G'day, your majesties."

Lynton Martin, 22, drove nine hours from Melbourne and donned a union flag print jacket and nine royal lapel pins before trying to catch a glimpse of the royals.

"I wanted to show that we are supportive and welcoming of the king," he told AFP, expecting an "aura" to Sunday's service.

Last year Martin traveled to London for Charles' coronation, which he described as a "spectacular" event.

During the church service, Bishop Christopher Edwards prayed for peace and an end to wars and asked that Charles' upcoming Commonwealth summit in Samoa be prosperous.

Later Sunday, Charles made a brief remarks at the New South Wales legislative council, where he hailed the "promise and power of representative democracy" and cracked a joke about his advancing age.

"I first came to Australia nearly 60 years ago, which is slightly worrying," he said to laughter.

"It just remains for me to say what a great joy it is to come to Australia for the first time as sovereign and to renew a love of this country and its people, which I have cherished for so long.".

Charles will spend the balance of Sunday at Admiralty House, a harborside mansion that is the Sydney residence of Australia's governor-general, the monarch's representative in the country.

Royal watchers eager to glimpse the king will have another chance on Monday, when he arrives in the capital Canberra alongside Queen Camilla for the busiest stretch of his slimmed-down schedule. 

Charles, who received the life-changing cancer diagnosis just eight months ago, is embarking on a nine-day visit to Australia and Samoa, the first major foreign tour since he was crowned. 

Visiting British royals have typically carried out weeks-long visits to stoke support, parading through streets packed with thrilled, flag-waving subjects. 

But the king's fragile health this time around has seen much of the typical grandeur scaled back.

Intentional or not, the more modest schedule should also help stave off Republican concerns about out-of-touch spending and lavish royal banquets.

Aside from a community barbecue in Sydney and an event at the city's famed opera house, there will be few mass public gatherings.

A handful of protesters gathered near the church on Sunday, brandishing demands to "decolonize" Australia.

Australians, while marginally in favor of the monarchy, are far from the enthusiastic loyalists they were in 2011, when thousands flocked to catch a white-gloved wave from Charles' mother, Queen Elizabeth II.