Is a fire extinguisher considered a hazardous material when transported as cargo?

In the United Kingdom, fire extinguishers are not considered as hazardous material for transportation by road.

Fire extinguishers are considered hazmat for transportation by road and sea and should be accompanied with Dangerous Goods Note and Material Safety Data Sheets.

Certain types of fire extinguishers may contain placard-required amounts of hazardous materials, including pressurized CO2 or nitrogen, liquids such as carbon tetrachloride, among other things. Therefore, the answer to the question would depend upon the specific type and quantity of extinguishers being transported.