A dangerous good is one that is capable of posing a significant risk to health safety or property when transported. If an item is flammable, radioactive, explosive, corrosive, an oxidising material, a compressed gas (such as aerosol can) or likely to deteriorate during its journey through the post, then you shouldn't be lodging these products at post offices or street posting boxes. However, Australia Post does accept limited quantities of some dangerous goods under contract conditions.

Goods Prohibited or Restricted in International Post
Many countries have restrictions on certain items such as medical samples and alcoholic beverages. Every country has a specific list of articles which are prohibited or restricted. For details on each country, refer to the International Post Guide

Goods Prohibited or Restricted within Australia
The delivery of plants, fruit, vegetables, insects, wildlife and firearms is prohibited or restricted within and between states in Australia.

Any non-letter article that requires carriage by air must bear a No Dangerous Goods Declaration.
Most prepaid Parcel Post, Express Post and Express Courier International satchels, boxes and parcel labels and service air bills have a no dangerous goods declaration area for senders to sign. Other services may need a declaration affixed to the item where carriage of the item may be by air.

If you are in doubt whether your item is a dangerous or prohibited good or want to find out more about sender's responsibilities, refer to the Dangerous & Prohibited Goods & Packaging Post Guide