Sydney (AFP) - King Charles III greeted throngs of admirers under the sails of the Sydney Opera House on Tuesday, the public finale to his first major foreign sojourn since falling sick with cancer.

The 75-year-old regent crisscrossed the city on his last full day of engagements, meeting lifeguards, sheep dogs, and cancer scientists before ending on the glistening waters of Sydney Harbour.

Toting umbrellas to shade themselves in the afternoon sun, thousands gathered outside the Opera House in the hope of snagging a brief encounter with the reigning monarch.

Freshly festooned with the honorary rank of navy admiral over the weekend, Charles sailed away from the Sydney landmark to inspect a flotilla of missile destroyers and coastal minehunters.

The one-time trainee jet pilot – who crashed an RAF plane in remote northwest Scotland – now holds five-star rank in each of the Australian service branches.

Brass bands piped out “God Save the King” before a fleet of helicopters and fighter jets buzzed the harbour in one final gesture.

“I feel good. I won’t wash that hand now for a while,” said Brian Webb, 69, a retired police officer, after meeting the king.

Police arrested a 60-year-old man outside the Opera House ahead of the king’s arrival, alleging he had been acting in “an abusive and threatening manner”.

The six-day trip, including three days of public duties, was trimmed back so it did not complicate Charles’ cancer recovery.

- Sausage sizzle -

Diagnosed with an undisclosed form of the disease in February, Charles did not address his illness publicly during the tour.

But nor did he totally shy away from it – stopping at a Sydney research facility where oncologists have been developing new ways to treat melanomas.

Lifeguards from Sydney’s Bondi Beach earlier watched as the “flexitarian” king – who gives up meat and fish two days each week – turned over sausages at a community barbecue.

“It’s a particular delight to see and smell all the top tucker here today,” he told the crowd, paying tribute to Australia’s much loved “smashed avo” and “cab sav” red wine.

“So thank you, everyone, for coming to join us at this ‘barbie’ today.”

In a makeshift field nearby, the king watched Australian working dogs compete to round up flocks of sheep.

His first trip to Australia as king – and the first visit by a reigning monarch in 13 years – Charles was always going to struggle to match the royal bonanzas of old.

It was at the Opera House in 1983 that the then-prince Charles and a 21-year-old princess Diana were mobbed by thousands of admirers desperate to see the “people’s princess”.

Australians 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.

Charles started the day in the inner-Sydney suburb of Redfern, the birthplace of one of Australia’s most significant Indigenous rights movements.

- Warmer welcome -

He received a warmer welcome than the previous day, when an Indigenous senator stunned guests inside Australia’s parliament by shouting at the king to “give us our land back”.

Charles first visited Australia as an awkward teenager in 1966, and has now set foot in the sun-kissed country on 17 separate occasions.

He regaled Australian lawmakers with stories of snakes, spiders and being made to eat the “unmentionable parts” of a bull calf.

“Hephner” the alpaca can now be added to that list, after sneezing over Charles as he stopped to shake hands outside the Australian War Memorial.

The first hiccup struck before the tour had even begun.

Plans to project a montage of images of Charles on the sails of the Sydney Opera House were briefly delayed because a cruise ship called the Queen Elizabeth was blocking the view.

Charles will depart Australia on Wednesday for the Pacific island nation of Samoa, where he will attend the annual meeting of the 56 Commonwealth nations.