Other than the odd error coin, there are no Australian general circulation decimal coins that could be considered to be rare.
They have not been around long enough to become rare and, in every year they were minted, there were millions, 10's of millions and even 100's of millions of each type minted.
Even the oldest of the Australian decimal coins are only just now reaching their maximum life expectancy of 40 to 50 years.
Ask again in 50 to 100 years and there may be a few rarities.
Indian head penny
Indian head penny
The rarest Australian coin minted for general circulation would be the 1930 Penny of which there were only 3,000 minted. It is currently esitmated that there are still about 1,500 of these still being traded in the market.
There are no Australian general circulation decimal coins that could be considered to be rare. They have not been around long enough to become rare and, in every year they were minted, there were millions of each type minted. Even the oldest of the decimal coins are only just now reaching their maximum life expectancy of 40 to 50 years. Ask again in 50 to 100 years and there may be a few rarities.
The rarest Australian Halfpenny issued for general circulation would be the 1923 Halfpenny. It is thought that there were only 15,000 minted. One of these coins, even in poor condition, might fetch over $1,000 AUD. The rarest of all Australian Halfpennies is the 1916 "mule". It has the reverse of the Australian Halfpenny and the obverse of the Indian Quarter Anna coin struck on an Australian Halfpenny blank (or planchet). These are an error coin and should never have made it into circulation. It is thought that less 10 were struck and are worth tens of thousands of Dollars AUD. All known examples of this coin are accounted for.
If you refer to general circulation Australian Pennies rather than Proof coins, the order runs something like this - 1. 1930 Penny is the rarest 2. 1920 Penny with no dots on the reverse 3. 1925 Penny 4. 1946 Penny
If your talking about the GBA games the rarest are Celebi, Mew, Jirachi and Deoxys until generation 4 (Pearl, Diamond and Platinum) Three new ultimate rares were introduced Shaymin, Arceus and Darkrai.
The rarest Australian Halfpenny is the 1916 (mule) coin, the 1923 coin is rare, and 1915, 1918 and 1939 (kangaroo reverse) coins are considered to be scarce.
In terms of minted as general currency, the Waitangi Crown of 1935 is definitely one of the rarest. Only 1128 of these were minted, and in good condition can be worth in excess of 7000 New Zealand dollars.
An Australian Threepence was a coin of silver appearance which was first minted in London in 1910 and last minted in Melbourne in 1964. From 1910 to 1944, their composition was 92.5% silver and 7.5% copper. They had a plain edge, weighed 1.41 grams and were 16 mm in diameter. From 1947 to 1964, their composition was 50% silver, 40% copper, 5% zinc and 5% nickel. They had a plain edge, weighed 1.41 grams and were 16 mm in diameter. All Australian coins have the reigning Monarch on the obverse, and the Threepence had the Australian "Coat of Arms" on the reverse with the word THREEPENCE around the top and the year at the bottom from 1910 to 1936. A new reverse design first issued in 1938 replaced the Australian "Coat of Arms" with three stalks of wheat with the year split either side of the stalks, AUSTRALIA across the top and THREEPENCE across the bottom. They were progressively withdrawn from circulation after the introduction of Decimal Currency on the 14th of February, 1966. In modern currency, a Threepence was the equivalent of 2.5 cents, but there was no equivalent decimal coin. There were no Threepences issued in 1913, 1929, 1930, 1931, 1937, 1945 and 1946. The rarest Australian Threepence is the 1937 pattern coin and the 1915 and 1921/22 overdate, the 1912, 1923, 1942 (Melb) and 1947 coins are considered to be scarce. From a coin collectors perspective, Australian Threepences, depending on year, condition and method of minting, can be worth anything from $1 to $200,000 AUD. Please note that since the Threepence is part of a redundant or obsolete currency, it no longer has an "exchange rate" with other currencies.
An Australian Sixpence was a coin of silver appearance which was first minted in London in 1910 and last minted in Melbourne in 1963. From 1910 to 1945, their composition was 92.5% silver and 7.5% copper (sterling silver). They had a milled edge, weighed 2.83 grams and were 19 mm in diameter. From 1946 to 1963, their composition was 50% silver, 40% copper, 5% zinc and 5% nickel. They had a milled edge, weighed 2.83 grams and were 19 mm in diameter. All Australian coins have the reigning Monarch on the obverse, and the Sixpence had the Australian "Coat of Arms" on the reverse with the word SIXPENCE around the top and the year at the bottom. They were progressively withdrawn from circulation after the introduction of Decimal Currency on the 14th of February, 1966. In modern currency, a Sixpence was the equivalent of 5 cents. There were no Sixpences issued in 1913, 1915, 1929 to 1933, 1937, 1947 and 1949. The rarest Australian Sixpence is the 1918 coin and the 1922, 1924, 1939, 1952 and 1953 coins are considered to be scarce. From a coin collectors perspective, Australian Sixpences, depending on year, condition and method of minting, can be worth anything from $1 to $145,000 AUD. Please note that since the Sixpence is part of a redundant or obsolete currency, it no longer has an "exchange rate" with other currencies.
Some continental coins were made of pewter, others of bronze or silver. The 1776 "continental currency" (dollar) is one of the rarest US coins and many were made from pewter, consequently suffering from corrosion of the tin.