Some Singtel customers were unable to connect to the Internet from home using fibre broadband yesterday following overnight maintenance.
The telco said about 3 per cent of its fibre broadband customers were affected.
Service was progressively restored through the day and was fully up in the evening, said Singtel, more than 12 hours after dozens of customers began reporting connection problems, starting at about 5am.
However, some customers said they were still not able to connect last night.
The Infocomm Media Development Authority (IMDA), Singapore's telecommunications regulator, said that it is investigating the disruption and will take enforcement action if there are any lapses.
It told Singtel to "restore services expeditiously" and update affected users on the recovery processes.
"IMDA takes a serious view of any service disruption to public telecommunications services, especially when many are home," it added.
Singtel acknowledged the downtime in a Facebook post at 7.50am. By 9am, almost 1,000 reports of outages had been logged on outage tracking website Downdetector.
This led to customers of Singapore's largest telco voicing their frustrations as they faced difficulties working from home (WFH) - the default mode of working here amid tightened Covid-19 measures. Many resorted to using their mobile data to connect online instead of using their home broadband and Wi-Fi.
In a Facebook update at about 6.15pm yesterday, Singtel told customers: "We recognise that it has been a trying time as many of you are working from home, and we are very sorry for the disruption."
The telco said its engineers are still investigating the cause of the issue, which affected customers in the central and eastern parts of Singapore.
"We are taking this incident very seriously and are taking steps to review our processes to prevent a recurrence," Singtel added.
Affected customers will be offered a 10 per cent discount on their monthly broadband subscription, which will be reflected in next month's bill.
Those who also have a Singtel mobile subscription plan and received an SMS from the telco will have their local mobile data charges incurred yesterday waived.
According to Downdetector, the number of reported outages for the telco peaked at around 1,500 just before 12.15pm.
A Singtel Facebook post on the matter at about 9.50am drew about 1,700 comments by last night.
While it is now the June school holidays, the downtime affected some who were still doing home-based learning.
Mr Felix Wu, 43, said that the outage made it difficult for his 10-year-old son to do his online holiday homework and he would have to catch up later.
The executive, who works in the healthcare sector, realised his home Internet was not working at 7am, and his connection was restored only at about 5pm. "The downtime was a bit too long," he said.