Charles III became King immediately after the passing of Queen Elizabeth II, but the King's representative Governor-General David Hurley made the Proclamation of Accession at a special meeting of the Federal Executive Council at Government House.

It later followed with a historic public reading of the proclamation of the new sovereign outside Parliament House, a First Nations' smoking ceremony, a 21-gun Royal salute and a stirring rendition of God Save The King.

"Because of the death of our blessed and glorious Queen Elizabeth II, the Crown has solely and rightfully come to Prince Charles Philip Arthur George," General Hurley proclaimed on Sunday.

"We therefore, general the honourable David Hurley, Governor General of the Commonwealth of Australia and members of the Federal Executive Council do now proclaim Prince Charles Philip Arthur George to be King Charles III by Grace of God, King of Australia and his Realms and Territories, head of the Commonwealth.

"And with hearty and humble affection, we promise him faith and faithfulness.

"May King Charles III have long and happy years to reign over us."

It is the first such pronouncement outside the current Australian Parliament and closely follows the official proclamation in London of the new British King at St James's Palace.

Flags will now return to full mast until dusk. The flags will then remain at half mast until the Queen's funeral on September 19.

"King Charles, of course, represents a new era. The second Elizabethan era has now passed. King Charles will need to forge his own path," Mr Albanese told the ABC's Insiders program.

"King Charles has been very active and outspoken on issues such as the need for the world to challenge climate change and to act on climate change."

But should he continue?

"In my view, that would be appropriate. That's a matter for him," the Prime Minister offered.

It comes as Mr Albanese announces a one-off "National Day of Mourning" public holiday in Australia on Thursday, September 22 to remember and pay respects to Queen Elizabeth.

A national memorial service, including a minute's silence, will be held in the Great Hall of Parliament House.

Mr Albanese, known to have strong republican views, praised the Queen for acknowledging that Australia was in charge of its own destiny. He regarded her as wise, encouraging and of "very good humour".

"70 years of devotion to duty, of loyalty and of love," the Prime Minister said.

"She had, I think, the capacity to deal with heads of state and she met with the British prime minister once a week of course. Over that time she met with 16 or she presided over 16 Australian prime ministers, 16 governors-general. That's a remarkable period.

"The longest serving ever British monarch, the second longest head of a sovereign state in the world's history is a remarkable achievement."

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The Prime Minister and General Hurley will travel to the United Kingdom to attend the Queen's funeral at Westminster Abbey.

Australia has offered London travel assistance to the leaders of New Zealand and 10 Pacific island states with deep British connections.

Mr Albanese has also confirmed that the four cancelled parliamentary sitting days which were to go ahead this week will now take place later this year.

The new dates have not been confirmed, but the Prime Minister notes at least one of those rescheduled days will be devoted to delivering condolences on the death of Queen Elizabeth.