Hi Anne, the ATO are a lot more understanding than you might think - there is no problem correcting your mistake!
As long as the net amount of the correction is less than $5000, then you can simply add it to your next BAS. If it is more, then you have to contact the ATO to do an adjustment to the BAS in which the error occurred. If you are using paper forms they can send you out an adjustment form to use, or if you are lodging online just follow the prompts to make an adjustment.
There is an ATO fact sheet that explains in detail how adjustments and mistakes are handled:
Fuel tax credits – making adjustments and correcting mistakes
The ATO also have a fuel tax calculator which can help you calculate your Fuel Tax credits. The great thing about this calculator is it also helps you work out any adjustments - just follow the prompts and when it asks "do you wish to correct a mistake" say "Yes". The following prompt - Do you wish to Claim EGCS answer "No", then fill in the number of litres etc. It will then guide you through how to complete your BAS.
Try the calculator here:
Fuel Tax Credit Calculator
Hope this helps!