The lives of millions of people in the densely populated Pearl River Delta are under threat after more than half a tonne of deadly cyanide was spilled into the river system's upper reaches.
Macau and Zhuhai, which draw all their potable water from the western tributaries of the Pearl River, have shut down their collecting systems and switched to reserves.
Authorities in both cities said they had enough water in their reservoirs to meet demand for about a month.
Hong Kong is expected to escape the direct threat of the toxic liquid because it draws supplies from the eastern part of the delta.
As a preventive measure, the Health Department said a test for cyanide contamination in goods imported from China would be added to the analyses carried out routinely at border crossings.
A truck carrying 200 cylinders, each containing 50 kilograms of sodium cyanide, overturned in the town of Wuzhou, on the border between Guangxi and Guangdong provinces, in the early hours of Tuesday morning.