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
20.00 dollars 1.00 twenty dollar bill 0.2 C-note
The Australian Twenty Dollar note is mostly red (with black, grey and white).
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.
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