https://docs.google.com/viewer?a=v&q=cache:Xb-1qdcKNkwJ:auspost.com.au/media/documents/dangerous-pro...

page 8 denotes fire extingiushers that are designated as being dangerous goods as "fire extinfuishers containing compressed or liquefied gas as Class 2 Non flammable, Non toxic gas..................

Empty non pressurized fire extinguishers do not contain compressed or liquefied gas and are not "dangerous goods"