You are liable to charge GST if you are registered, or required to be registered, and you make a taxable supply.

You make a taxable supply when:

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you are registered or required to be registered;
you make a supply for consideration;
the supply is connected with Australia;
the supply is not GST-free or input-taxed
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The definition of supply catches just about everything, so unless you’re making a GST-free supply or an input-taxed supply, and you are registered, then you most likely have to charge GST.

To work out the amount of GST you would charge, you add 10% of the sale price.