Charles is undergoing cancer treatment and doctors have advised him against any further extension of the trip, so neighbouring New Zealand will miss out on a royal visit.
The Australian leg of the trip, at the invitation of the federal government, will see the royal couple carry out engagements in the Australian Capital Territory and New South Wales.
In March, Prime Minister Anthony Albanese said plans were under way on logistics for a possible visit.
"The King has shown his compassion for Australians affected by recent natural disasters, just as Australians have shown compassion and support for the King following his cancer diagnosis," he said.
"The King, Queen and members of the Royal Family are always welcome in Australia."
In Samoa, they will celebrate "the strong bilateral relationship" between the Pacific Island nation and the UK, and the couple are expected to attend the Commonwealth Heads of Government Meeting 2024.
The meeting brings together delegations from 56 countries in Africa, the Caribbean and Americas, Europe, Asia, and the Pacific.
The visit would be the first trip to Australia by a reigning monarch since 2011.
King Charles has made 15 visits to Australia, his last in 2018 to open the Commonwealth Games. (AP PHOTO)
The last such visit was when Queen Elizabeth II travelled to Canberra, Brisbane, Melbourne and Perth.
King Charles has made 15 visits to Australia, his last being in 2018 to open the Commonwealth Games on the Gold Coast and to visit parts of Queensland and the Northern Territory.
Buckingham Palace says the couple's program will be subject to doctors' advice, with any necessary modifications to be made on health grounds.
Further details of the visit to both countries will be announced soon.
Charles, 75, returned to public-facing duties earlier this year while still receiving treatment for an undisclosed form of cancer.
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