Can the tax man find out part of my income & tax payable?

I have an ABN for cleaning services, and sometimes disclose it when invoice is required, which means I will need to pay tax for it. But I have had a regular customer for many years, I've never provided them invoices, and they never asked for one. The regular amount of money would be transfered periodicaly to my personal bank account from their company account, without withholding any tax, and I've never paid any tax by myself. Financial year is coming, and I'm a bit worried!