The Queen has opened a new wing of Australia's Sydney Opera House - which she called "the symbol of a nation".
The Queen opened the Sydney Opera House in 1973
The newly opened western side of the site features large windows providing views of Sydney's harbour and skyline.
The Queen opened the building in 1973, and returned with Prince Philip during a five-day visit to Australia in which she will open the Commonwealth Games.
They were greeted by about 2,000 people who cheered and sang along when God Save the Queen was played.
A 21-gun salute boomed across Sydney harbour while the Queen inspected a guard of honour before meeting political dignitaries.
She said: "When in October 1973 I opened this building, it was universally agreed that the Opera House was something more than a performing arts centre, more than a great work of architecture.
"It was seen even then as, and has certainly since become, the symbol of the nation itself - a building to which visitors happily return again and again for renewed joy and inspiration."
And the monarch heaped praise on the original architect Jorn Utzon, who designed the new feature with his son Jan.
"It confirms that the Opera House is not something sacred, but a living structure, a vibrant and evolving place that meets the needs of its users and reflects the wishes of the people," she said, commenting on the new development.
Mr Utzon stopped working on the building in 1966 following a series of disagreements with the then state government of New South Wales.
His son, Jan, spoke at a reception for the Queen:
"My father, who is turning 88 next month, sends his warmest wishes and greetings to everyone here," he said.
He added that his father, who could not make the journey to the building, took "much pleasure" from the Queen's opening of the new development.
The Queen, who turns 80 this year, later attended a Commonwealth Day service in Sydney - the first time the event has been held outside the UK.
The day was commemorated with a service of song, dance and prayer at St Andrew's Cathedral.
The theme of this year's day is Health and Vitality: the Commonwealth Challenge.
In her Commonwealth Day message, played to the congregation, which included Australian Prime Minister John Howard, the Queen said: "Good health is a precious gift.
"Yet many do not share in this."
She praised international action taken on polio and hoped the same success could be achieved with diseases such as malaria and tuberculosis.
The Queen was due to leave Sydney for Canberra later on Monday.