King Charles says he and Queen Camilla are "utterly shocked and horrified" by the "senseless attack" in Sydney.
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The King added that the royal couple's "hearts go out to the families and loved ones of those who have been so brutally killed".
In a personal social media post, Prince William and his wife Kate said they were "shocked and saddened" by the stabbings, adding that their thoughts are with those affected and the "heroic emergency responders who risked their own lives to save others".
It has been reported that Charles plans to tour Australia later this year, accompanied by Camilla.
The King's most recent visit to the country was to open the Gold Coast Commonwealth Games in 2018.
His first official visit to Australia was as his mother's representative at the memorial service of prime minister Harold Holt in 1967
Charles has made a total of 16 visits to Australia, on 15 of which he either accompanied or represented his mother Queen Elizabeth II.
In a statement, the King said: "My wife and I were utterly shocked and horrified to hear of the tragic stabbing incident in Bondi.
"Our hearts go out to the families and loved ones of those who have been so brutally killed during such a senseless attack.
"While details of these shocking circumstances are still emerging, our thoughts are also with those who were involved in the response, and we give thanks for the bravery of the first responders and emergency services."
In a personal social media post, William and Kate wrote: "We are shocked and saddened by the terrible events in Sydney earlier today.
"Our thoughts are with all those affected, including the loved ones of those lost and the heroic emergency responders who risked their own lives to save others. W & C"
The royal family has a long association with Australia and has forged close links with its people and institutions during many trips to the country.
Charles and Diana, Princess of Wales, toured the country in 1983 and were pictured with their son William, then just an infant.
Eleven years later when Charles visited Sydney, he witnessed dramatic scenes when a student was wrestled to the ground after firing a starting pistol as the royal stood to make a speech.
Between 1954 and 2011, Queen Elizabeth II visited Australia 16 times, including a 1973 trip in which she opened the Sydney Opera House.
Australian Associated Press
