If you are an employee of one or more employers who submit your income information to the ATO on your behalf and you only have some simple deductions to claim, then that would require a simple tax return.
People who only receive government payments (age pension, youth allowance, etc) will also generally require a simple tax return.
Things get a little more complicated when you have:
If any of these apply, then more work is required by your accountant and therefore it will not be considered as a simple tax return.
If there are extensive work-related deductions, these too will push the preparation of a tax return beyond what is a simple tax return.