The joke answer is Ten Dollars. Value as a collectible is zero. If you have the other half, a bank will give you Twenty Dollars for it.
it is worth much more
Australian twenty-dollar note was created in 1966.
It seems that any note with a Star is worth a little more than face value. Especialy if it is an older note. The best place to shop for prices is on Ebay.
As of 14-Dec-2009, £20 GBP is equivalent to $35.71 AUD, or about half way between a Twenty Dollar note and a Fifty Dollar note. Take your pick. It will be different tomorrow.
Twenty dollars
Twenty Australian dollars
Jackson
While the appearance of newer twenty dollar notes has changed, the 1993 $20 bill is, like more recent issues, a Federal Reserve Note and remains worth twenty dollars as legal tender in the United States. We know of no special or rare printings of the 1993 that might make a specimen worth more than its twenty dollar face value to collectors.
The Australian Twenty Dollar note is mostly red (with black, grey and white).
20.00 dollars 1.00 twenty dollar bill 0.2 C-note
This is a Federal Reserve note and should have a green seal. They are worth $22-$30 in circulated condition and $40 or more in uncirculated condition.
cheak on Museum of Australian Currency Notes