SYDNEY: Australia was forced to reassure its citizens that their personal data was secure on Wednesday, after malicious attacks by overseas hackers prompted an embarrassing shutdown of the online census. Australians are required to complete the census every five years or face fines, and this was the first time there had been a major push for people to fill in the survey online. But as thousands of people headed to the official website Tuesday evening, a series of denial-of-service attacks — attempts to overwhelm an online system to prevent people accessing it — prompted authorities to take the site offline. “It was an attack, and we believe from overseas,” said David Kalisch from the Australian Bureau of Statistics, which organizes the census. “The scale of the attack, it was quite clear it was malicious,” he told the Australian Broadcasting Corporation. The census website was not back online on Wednesday.