The coin will have its date, denomination and weight on it. The condition will play a role in its value. Most are based on bullion. Kitco.com posts up to date prices of precious metals. Southern California Stamp and Coins will value it for free. They're reputable and give no pressure.
It depends what 2009 gold Australian coin that you're talking about. The Perth Mint strikes several gold coins for Australia. They have shark coins, they have an eagle coin, kangaroo, and a few others. Depending on how many were struck and the availability the coin can be worse more than its weight in melt or less. Having it graded and what the grade is also can decide its value.
Had the coin been graded by a grading service like pcgs or NGC? If not that is the first step.
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