This is a beautiful and impressive potter wasp, ranging from 25 to 29mm; males are smaller and thinner. Brilliantly marked with yellow and black. The yellow pattern on the thorax distinguishes it from other species. The petiole (the "waist" joining thorax and abdomen) is also very long, even by potter wasp standards.
This species is sometimes listed as Phi flavopunctatum in Hong Kong and Mainland Chinese publications; this is an outdated name. I have also seen it featured on a stamp in Singapore, and referred to as "Delta arcuata". I have no idea about the validity of this name, but since it appears to have structural characteristics distinguishing it from members of the genus Delta, I will stick with the current name. It can be found in Hong Kong, Taiwan and parts of China and Thailand. I did not expect to find it as far south as Singapore, but it is quite surprisingly common there.
In Hong Kong, this species is particularly active during late autumn to mid-winter (November, December and January). It also appears during parts of spring and summer. It is not particularly common but can be seen sporadically in rural and countryside areas. In Singapore, this species can be seen feeding on flowers of the mango tree (one of the most common trees, planted across many housing estates) or building its round nests under roofs, eaves and other parts of buildings.
Despite its impressive appearance, this species, like all potter wasps, is totally harmless and will not attack at any time. If you happen to find one in the process of building its nest, I suggest you take the chance to watch this fascinating process. Just don't get too close, not for fear of being attacked but to prevent scaring it away!
The first photo is of an individual in Hong Kong. The second and third were taken in Singapore.