The following photo shows a typical water tap found in old HDB flats.
After much searching, I finally found an old water tap key that fits. It is hollow in the middle.
The old water tap key allows the tap's shaft to go through it. Some filing of the shaft might be required to ensure the key is able to grip the shaft.
The shop that I got it from is "Ng Hoe Hardware & Paints Shop" at Jurong East Street 24. It is probably the only shop in Singapore that still sells this type of water tap key.
