Agree with RedMask. The use of STG as an abbreviation for sterlking was/is quite common. There is no formal hallmarking system/register in Australia and thus no register of maker's marks. However, in a country with some unique fauna it is not uncommon for an Australian piece to be punched with an animal mark.
There is a great book by Christine Erratt that probably contains the answer (sorry for the link http://www.ascasonline.org/articoloAGOST134.html
) but I don't have a copy. You could try dropping the people at the Silver Society of Australia to see if they can help (http://www.silversociety.com.au/index.php?id=4
) - I used to be a member but no longer live close enough to participate in the Society.