Do you only do tax return when your income is over $18,000?
That depends on your legal status - are you a sole trader or a company? The 18k tax free threshold only applies to individuals, and it's real intention is to minimise the paperwork for employees who didn't earn much. If you've got a business, particularly if the REVENUE is over 18k, you should be lodging a tax return.
If you're a company then it's much clearer - do your taxes.
And if you don't need to lodge a tax return, you still need to submit a 'Non-Lodgement Advice Form' to the ATO so that they don't list as you being delinquent and start chasing after you.
It's also important if you are making a loss. They are carried forward and can be recouped when you are earning - rules apply, and you're better off talking to an accountant about the best way to use them for tax purposes. I'd also talk to an accountant to make sure you are claiming all your deductions.
What Dionne has written is correct. If you are a partnership you have to consider if the other partner needs to lodge their tax return. If you're operating through a trust you definitely have to lodge - and have an accountant on board!
Good luck with your business.