The 21/2-hour power failure at the Sha Tin switchboard station, one of three SmarTone has across the city, affected 25 per cent of its cell sites, it said in a preliminary report submitted to Ofca.

Cell sites are locations where antennas and other electronic communications equipment are placed. The operator did not reveal the total number of affected customers.

'It is difficult to estimate ... due to the mobility of users and the lack of [previous] reference to a non-working day within a long holiday,' the report said.

Ofca said it would ask the firm to clarify the number.

Charles Mok, founding chairman of the Internet Society Hong Kong, said he believed SmarTone did not conceal anything at this stage. But he felt the report failed to explain the reason behind the breakdown.

The incident began when a power failure hit the building containing the switchboard station at 8am, the report said. A standby battery kicked into operation immediately, and 15 seconds later, the back-up generator took over from the battery, it said.

The battery could last for four hours, while the generator could last as long as it had fuel supply. But the generator unexpectedly stopped and restarted three times within a minute at 10.35am, causing the starter component to burn out.

At 12.30pm, Ofca approached the firm upon receiving complaints, the report said. According to guidelines, the operator should have reported to Ofca within an hour of any incident taking place on a public holiday.

The report said SmarTone's hotline and retail employees were informed at 11.01am, while customers were notified through Facebook at 2.13pm, by which time services had almost returned to normal. It said voice, mobile and internet services largely resumed at 1pm, and the SMS at 2.30pm.

It acknowledged that the announcement should have been posted as soon as the disruption occurred. But the person in charge of customer communication wanted to gather more details to provide a more complete explanation to users.

The naming of the Sha Tin switchboard station was at odds with Ofca's comments to media on Monday that the affected centre was in Central and Western District. Mok said the discrepancy could be due to miscommunication between the two bodies.

The report also noted 'unsubstantiated media reports' on the station's location, duration of the disruption and number of affected customers. It denied the upgrade of its 4G network caused the breakdown.

The firm must submit a full investigation report to the office by April 27. Ofca will study both reports to decide if the operator has breached any licensing conditions.

$6.24b

SmarTone's sold this amount of mobile phone service plans in the year to June, 2011