Well I found another one similar except this time the phrase used is "stamp duty".

The term "duty" I can easily understand as a tax on imports. Therefore the stamp signifies that the tax has been paid. From what I can tell the cancellation on the second stamp refers to the shipping company used to transport the imported item. I believe the company to be the Pacific Steam Navigation Co. which later became the Royal Mail Steam Packet Company and operated a route from Sydney to London. The phrase on the cancellation appears to be slightly different than either of those two names so I am not 100% sure.