There was nothing systemic or institutional about the breach
Passport Numbers and Visa details of G20 world leaders were emailed to Asian Cup Organizers in Australia due to human error before the G20 summit in Brisbane in November 2014. The sender forgot to uncheck the autofill option which resulted in sending to unintended receiver. Also to be noted is that, the emails were said to be deleted within 10 minutes and no copy were stored on their servers.
Given that the risks of the breach are considered very low and the actions that have been taken to limit the further distribution of the email, I do not consider it necessary to notify the clients of the breach
It should also be noted that the personal details of these individuals, including their names, positions and dates of birth are generally already available in the public domain given their prominent positions.
But probably not visa details.