Depends on your background. Are you already in accounts/bookkeeping/accounting? If so then a MYOB course would be handy and maybe even a Quickbooks course just so you can attract a few extra clients. If you have not really dealt a lot with GST or payroll then a 1 day taxation course which comes with a user friendly manual would be needed.

If you have little or no accounts background, then I would suggest finding something else and doing a course at Tafe or a couple of basic accounting subjects at Uni. HTH