12.5% rate (pre 1/10/2010)
    - Inclusive amount - divide by 9           = GST amount
    - Inclusive amount - multiply by 8/9    = exclusive amount    

15% rate (after 1/10/2010)
    - Inclusive amount - divide by 7.6666    = GST amount or
    - Inclusive amount - multiply by 3/23     = GST amount
    - Inclusive amount - multiply by 20/23   = exclusive amount
    
Example 15% rate
                    $115 GST incl. x 3/23          =    $15 (GST)
                    $115 GST incl. x 20/23        =    $100 (GST exclusive amount)
                    $100 x 15%                           =    $15 (GST)

For an example calculation of one off GST Payments Basis made 30/09/2010, click here...