Australian Pennies have never had a Roman soldier on them. You are describing a British Penny, and the "Roman soldier" is the goddess Britannia who personifies Britain. Britannia has appeared on various British coins for hundreds of years.
Britannia was the ruler and Britain was Scotland, wales, England, Cornwall and Ireland all put together.
The only George III coins with Britannia on the reverse are the Twopence, Penny, Halfpenny and the Farthing, but none of them had a milled edge. From 1797, the Penny was 36mm in diameter, the Twopence was 41mm in diameter and both coins were made from copper.
The British "NEW PENNY" was first issued in 1971 and featured Queen Elizabeth II on the obverse with a crowned portcullis on the reverse and was 20.32 mm in diameter. The 1871 Penny featured Queen Victoria on the obverse with Britannia on the reverse and was 30.81 mm in diameter.
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1. British, not Canadian 2. Standard size for a pre-1971 British penny, not an extra large cent. 3. The goddess Britannia, not a Roman warrior. Assuming that at least the date is as stated, your coin is common among collectors and retails for about a quarter.
What penny! :) Ebay most likely. Put on the description "lucky penny"
As with most British pre-decimal Pennies for the past few hundred years, there is the seated figure of Britannia carrying a Trident and Shield. The various images of Britannia are mostly very similar but for minor variations. See the link below.
Most years of issue of the British Queen Victoria Penny could be described as a "Passion Penny", some more so than others. The title "Passion Penny" arises from the proximity and angle of the shaft of the trident she is holding in relation to her groin. The humour of the "Passion Penny" design must have worn thin after 50 odd years, and from 1895 the trident shaft was moved away from the groin, almost to the knee. For a valuation of a British Penny, or any other coin, please specify the year.
The only 1857 coin with Queen Victoria and Britannia on opposite sides and roughly the size of a Penny, is a Penny.
The Roman Empire collapsed in the year 476 so you're only off by 1,400 years or so :-) What you have is a BRITISH penny with a picture of the goddess Britannia on the front. See the Related Question for more.
It will clean the penny. :)