Singtel has been fined $500,000 - the largest amount in three years - for an islandwide fibre broadband outage last year that left some 490,000 users cut off from the Web.
The outage on Dec 3 lasted nearly 24 hours and affected close to 90 per cent of Singtel's fibre broadband user base.
Announcing the fine yesterday, the Info-communications Media Development Authority (IMDA) said its investigations showed that the disruption was triggered by a planned maintenance.
The exercise to install security patches overloaded the servers, which could not process subscribers' request for connection to any website.
IMDA also noted that there were warning signs prior to the incident, with the same servers running at almost full capacity - up to 90 per cent - but Singtel "had failed to take prompt action".
In view of the high utilisation rate, Singtel should have exercised "greater due diligence and caution" to prevent overloading the servers when installing the security patches.
Number of users cut off from the Internet in an islandwide fibre broadband outage last year.
In determining the fine, IMDA took into consideration mitigating factors such as the $5 million in discounts and bill waivers Singtel offered its customers in compensation.
Affected customers who were told to surf the Web using their mobile connection had their mobile data charges waived.
The telco also extended a 10 per cent discount to customers' fibre broadband bill for the whole of December.
"We deeply regret the fibre broadband service disruption last December. We know how important network reliability is to our customers, and we have learnt from this incident," said Mr Yuen Kuan Moon, Singtel Consumer Singapore's chief executive officer.
Singtel has also since upgraded its servers to prevent a similar incident from recurring, and is reviewing its broadband network architecture to improve its resilience.
The largest telco fine of $6 million was meted out in May 2014 to Singtel over a fire at its Bukit Panjang Internet exchange in 2013 that took down phone lines, banking payment and Internet services across Singapore.
In that case, the unprecedented islandwide outage had affected close to 270,000 subscribers, including DBS branches and ATMs, SingHealth polyclinics, Singapore Pools branches and AXS payment machines.
Home fibre broadband users of Singtel, StarHub and M1 were also cut off from the Internet, as the fire damaged the cables of national fibre broadband network builder Netlink Trust that were housed in the same premises.