Mintmarks, if they are used at all, could be almost anywhere on predecimal (pre-1966) Australian Coins. You would need to be quite specific about a particular coin in a particular year.
Mintmarks, if they are used at all on Australian decimal coins, constitute minor alterations to parts of the animal on the reverse design.
If you are keen to find out, I can recommend the following book which contains description and pictures showing mintmarks and where to find them.
Pocket Guide to Australian Coins and Banknotes by Greg McDonald.
Australian coins minted at the Melbourne Mint do not have a mintmark.
All 1988 Australian coins were minted at the Royal Australian Mint Canberra. Since they were all minted at the same place, there is no mintmark.
Any 1942, 1943 or 1944 Australian Threepence, will have the "S" (San Francisco) or "D" (Denver) mintmark on the reverse of the coin, on the right hand side, just below the fourth digit of the year. Any coins minted in Melbourne will have no mintmark.
You do not specify a denomination or a year. Australian predecimal coins minted at the Royal Mint London prior to 1950, have no mintmark. Australian Halfpennies and Pennies minted at the Calcutta Mint in India in 1916 to 1918 inclusive, have an "I" mintmark below the bottom scroll on the reverse of the coins. Australian Halfpennies and Pennies minted at the Bombay Mint in India in 1942 and 1943, have an "I" mintmark below the head of George VI on the obverse of the coins.
Australian Threepences minted from 1910 to 1915 were minted at the Royal Mint London and have no mintmark. Australian Threepences minted from 1916 to 1926 were minted at either the Sydney Mint or the Melbourne Mint. Sydney Mint coins have no mintmark, the coins minted at the Melbourne Mint and have an "M" below the date. Australian Threepences minted from 1927 to 1941 were minted at the Melbourne Mint and have no mintmark. Australian Threepences minted from 1942 to 1944 were minted at the Melbourne Mint, the San Francisco Mint (USA) and the Denver Mint (USA). The coins minted at the Melbourne Mint have no mintmark. Coins minted at the San Francisco Mint will have an "S" below the units digit of the date. Coins minted at the Denver Mint will have a "D" below the units digit of the date. Australian Threepences minted from 1947 to 1964 were minted at the Melbourne Mint and have no mintmark. Some Australian Threepences minted in 1951 were minted at the Royal Mint London. The mintmark is "PL". The "P" appears above the first "E" in "THREE", the "L" appears above the "E" in "PENCE".
From its initial issue in 1910, until 1915 inclusive, the Australian Shilling was minted at the Royal Mint London and has no mintmark. Some of the 1915 issue of the Australian Shilling were minted at the Heaton Mint in Birmingham and have a mintmark "H". From 1916 to 1926 inclusive, the Australian Shilling was minted at either the Royal Australian Mint Melbourne or Sydney. Coins minted at the Melbourne Mint have a mintmark "M", coins minted at the Sydney have no mintmark. With a few exceptions, from 1927 to the last minting in 1963, the Australian Shilling was minted at the Royal Australian Mint Melbourne. Coins minted at the Melbourne Mint have a mintmark "M" if they have a mintmark at all. From 1942 until 1946 inclusive, Australian Shillings not minted at the Melbourne Mint exclusively, were minted in addition to the Melbourne coins at the following Mints - 1942 - San Francisco Mint "S" 1943 - San Francisco Mint "S" 1944 - San Francisco Mint "S" 1946 - Perth Mint - small dot (.) before the "S" in "SHILLING" (.SHILLING)
Not all coins need a mintmark. All Australian 1919 Pennies were minted in Melbourne. For reasons that are not explained, most 1919 Australian Pennies had no mintmark, others had a dot above the top scroll and below the bottom scroll, and others had a dot below the bottom scroll.
Fijian coins have been minted variously at the following mints - Royal Australian Mint, Canberra - mintmark = C The Mint, Birmingham (Heaton) - mintmark = H The Royal Mint, Llantrisant, Wales - mintmark = L Royal Canadian Mint, Ottawa - mintmark = O San Francisco Mint, USA - mintmark = S
From its initial issue in 1910, until 1915 inclusive, the Australian Threepence was minted at the Royal Mint London and has no mintmark. From 1916 to 1926 inclusive, the Australian Threepence was minted at either the Royal Australian Mint Melbourne or Sydney. Coins minted at the Melbourne Mint have a mintmark "M", coins minted at the Sydney have no mintmark. With a few exceptions, from 1927 to the last minting in 1964, the Australian Threepence was minted at the Royal Australian Mint Melbourne. Coins minted at the Melbourne Mint have a mintmark "M" if they have a mintmark at all. From 1942 until 1951 inclusive, Australian Threepences not minted at the Melbourne Mint exclusively, were minted in addition to the Melbourne coins at the following Mints - 1942 - San Francisco Mint "S", Denver Mint "D" 1943 - San Francisco Mint "S", Denver Mint "D" 1944 - San Francisco Mint "S" 1951 - London Mint "PL"
KG is not a mintmark, it is the initials of the designer of the (kangaroo) reverse of the Australian 1938 to 1964 Penny and Halfpenny coins, George Kruger Gray. 1940 and 1941 Pennies minted at the Perth mint have a dot between the K and the G (K.G) as the mintmark.
The 1944 Australian Penny was minted at one of two mints. Melbourne Mint (no mintmark - 2.45 million minted. Perth Mint (mintmark = Y.) - 27.83 million minted.
British Edward VII (1902-1910) Sovereigns and Half-Sovereigns were minted at the Royal Mint London, Ottawa, Melbourne, Perth and Sydney Mints. The coins minted in London have no mintmark. The Sovereign and Half-Sovereign coins minted elsewhere have a mintmark "on the ground" below the horse and above the date. The coins minted in Ottawa have a mintmark "C". The coins minted in Melbourne have a mintmark "M". The coins minted in Perth have a mintmark "P". The coins minted in Sydney have a mintmark "S". The letters BP to the right of the date is the designers initials (Benedetto Pistrucci) who designed the St George and the Dragon scene.