Hang Seng Indexes, which manages the Hang Seng group of indices, said the technical glitch in its system which disrupted trading late last month was caused by an unusual sequence of events.
Updating of major indices for half an hour on January 22 was disrupted by the glitch and the group said it was carrying out a study to identify possible areas for improvement.
In a report to the Legislative Council's financial affairs panel, Hang Seng Indexes said the factors that caused the problem were so rare that it could not have been foreseen.
'We're talking about something that happened within a thousandth of a second, or even less,' Vincent Kwan Wing-shing, director of Hang Seng Indexes, said.
Kwan said the incident was the first major technical mishap that had occurred in the system, which remained 'highly resilient'.
The glitch meant that the Hang Seng Index and the Hang Seng China Enterprises Index could not be calculated properly from 10am to 10.32am, although individual share prices continued to function.