Not enough information.
Has it been circulated? Is there anything unique about it? Whose signature is on the front? What is the serial number?
Notes printed from 1966-1972 will have "COMMONWEALTH OF Australia" printed at the top, front and back.
Notes printed from 1972-1994 will have only "AUSTRALIA" printed at the top, front and back.
For a reasonable valuation to be given, the Serial Number and Signatories would need to be known.
The Commonwealth of Australia $20 note was printed in 1966, 1967, 1968 and 1972.
A Commonwealth of Australia $20 note uncirculated and in absolute mint condition could fetch anything from $110 to $500 AUD. The 1967 note could fetch up to $8,000 AUD.
If it has been circulated but still in good condition, it might fetch anything from $28 to 425 AUD.
A reputable coin dealer will be able to give a more accurate valuation.
Australian banknotes with the legend "Commonwealth of Australia" were minted from 1966 to 1972.
A Twenty Dollar with the legend "Commonwealth of Australia" may have been printed in 1966, 1967, 1968 or 1972.
The signatories and serial number ranges were -
Coombs/Wilson - XAA to XBP in 1966
Coombs/Randall - XBQ to XBS in 1967
Phillips/Randall - XBS to XEU (and maybe ZXA) in 1968
Phillips/Wheeler - XEV to XGY in 1972
To get a valuation on an Australian banknote, would need all of the above information and the condition of the note.
The Royal Australian Mint never issued a Twenty Dollar coin until the 1990's and then usually in a mint set of bullion coins.
The first Australian Dollar coin was issued in 1984.
There were no 1968 Australian Five Dollar notes printed.
There were no Australian Fifty Dollar notes printed in 1984.
Twenty Australian dollars
The Australian Dollar was going for about $0.73 USD in late 1994. The Australian Dollar is currently going for about $0.93 USD as of 3-Dec-2009.
The first Australian Two Dollar coin was issued in 1988.
There were no 2003 Australian Ten Dollar star notes issued.
The conversion rate between the Australian dollar and the Indian rupee is one Australian dollar to 56.42 rupees. One rupee equals 0.02 Australian dollars.
Such a coin does not exist. The Australian Two Dollar coin was first issued in 1988.
The first Australian Two Hundred Dollar coins were issued in 1980.
Such a coin does not exist. The Australian One Dollar coin was first issued in 1984.