A choked drain in a nearby construction site for the upcoming Upper Thomson MRT station led to the flash flood which put a dampener on the festive mood at several eateries on Christmas Eve.
This was uncovered during investigations by national water agency PUB.
To fix the problem, the Land Transport Authority (LTA) will get the contractor of the worksite to further improve the flow of water in the temporary diversion drain - which acts as a conduit for rain water while drainage upgrading works are ongoing.
"As part of works to construct Upper Thomson MRT station, PUB is also working with LTA to upgrade the drain along Upper Thomson Road to enhance flood protection for the area," the authorities told The Straits Times.
PUB has also reminded construction-industry players to conduct checks on the public drains in the vicinity of their respective construction sites in preparation for the monsoon season. The flash flood incident took place last Saturday at a row of shops between Jalan Keli and Jalan Todak near Thomson Plaza.
It was not just soaked diners who had to wade through knee-high waters after rain water flooded the eateries.
About eight businesses along Upper Thomson Road were affected by the flash flood, and tenants said the incident caused them to rack up thousands of dollars in losses.
The Meteorological Services Singapore said that average rainfall over Singapore that day was 35.5mm, with the highest daily rainfall that day recorded in Tai Seng, where 86mm of rain was recorded.
For Mr Jamal Mohamed, 50, a cashier at The Roti Prata House - one of the affected eateries - his loss estimate was $30,000, including the hours that the shop had to stay closed while workers cleaned up.
After closing at about 5.30pm on Saturday, workers managed to keep regular business hours only on Monday.
"Everything that was placed nearer to the floor was affected. We salvaged what we could, but flour, masala, rice and eggs were wasted," said the employee, who has been working at the food joint for nine years.
When The Straits Times visited the area on Wednesday, Mr Nicholas Lim, 19, a part-time employee at ice cream shop Udders, had cleaned the pavement outside the shop, and "cordoned" it off using chairs.
"The drain overflows with sandy water, so the pavement becomes sandy. I've been trying to get rid of the sand," he said.
The nearby construction for the Upper Thomson station, part of a 22-station Thomson Line, started in 2014.
The station will be built underneath a stretch of Upper Thomson Road between Thomson Plaza and the Church of the Holy Spirit.
Flash floods have occurred on 10 days this year, including Christmas Eve, as of Wednesday.
This is up from the six during the El Nino year last year, but down from the 17 flash-flood days recorded in 2014.
PUB has undertaken efforts to reduce flood-prone areas in Singapore, including the widening and deepening of drains and canals, the construction of new drains and the upgrading of ageing ones.
Such efforts have helped reduce the flood-prone areas in Singapore to 30.5ha - down from the 35ha last year.